The parking brake assembly is designed to apply the brakes mechanically to prevent the car from rolling when parked, or to stop the car in the event of a complete hydraulic failure. Most parking brakes operate on the two rear brakes. Some vehicles with front wheel drive have front wheel parking brakes. In these cases, in an emergency stop, most of the stopping power would be required on the front of the car.
The parking brake may be activated by a hand lever or a foot pedal. In either style, pushing the pedal or pulling the lever causes a cable connected to the rear brakes to be pulled. The cable has an equalizer so that the cable pulls the same amount on both left and right rear brakes.
The parking brake lever is connected to the secondary brake shoe. The lever is mounted on the back of the shoe and is connected to it by a pivot pin located in the upper end of the lever. The pivot pin is retained in the shoe by a washer and clip. The parking brake cable is attached to the lower end of the lever. A strut, located below the lever pivot pin, connects the lever to the primary brake shoe. The strut is notched at each end and is fitted into accommodating notches in the lever and primary brake shoe. An oval-shaped spring, installed on the primary shoe end of the strut, is used to position the strut.
When the parking brake is applied, the cable pulls the lower end of the parking brake lever forward, causing the connecting strut to push the primary brake shoe forward. At the same time, the upper end of the lever pushes the secondary brake shoe rearward. The combined action of the lever and strut expands the brake shoes, forcing them against the drum to develop brake action.
Showing posts with label Brakes. Show all posts
Showing posts with label Brakes. Show all posts
Thursday, January 31, 2008
Repair Guide: Disc/Drum Brakes
Disc brakes are used on the front wheels of most cars and on all four wheels on many cars. A disc rotor is attached to the wheel hub and rotates with the tire and wheel. When the driver applies the brakes, hydraulic pressure from the master cylinder is used to push friction linings against the rotor to stop it.
The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.
A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.
The hydraulic and friction components are housed in a caliper assembly. The caliper assembly straddles the outside diameter of the hub and rotor assembly. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.
Most late-model cars use the moving caliper design. This design uses a single hydraulic piston and a caliper that can float or slide during application. Floating designs 'float' or move on pins or bolts. In sliding designs, the caliper slides sideways on machined surfaces. Both designs work in basically the same way.
The single-piston caliper assembly is constructed from a single casting that contains one large piston bore in the inboard section of the casting. Inboard refers to the side of the casting nearest the center line of the car when the caliper is mounted. A fluid inlet hole and bleeder valve hole are machined into the inboard section of the caliper and connect directly to the piston bore.
The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.
The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.
The caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper. Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Since the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.
The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.
As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.
Sliding calipers are made to slide back and forth on the steering knuckle support to which it is mounted. There is a V shaped surface, sometimes called a rail, on the caliper that matches a similar surface on the steering knuckle support. These two mating surfaces allow the caliper to slide in and out. The internal components of the caliper are the same as those previously described.
The stationary or fixed caliper has a hydraulic piston on each side of the rotor. Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.
Question:
My brakes are squealing. Does that mean I need a brake job?
Answer:
Not necessarily. A certain amount of brake noise is considered "normal" these days because of the harder semi-metallic brake pads that are used in most front-wheel drive cars and minivans. This type of noise does not affect braking performance and does not indicate a brake problem. However, if the noise is objectionable, there are ways to eliminate it.
Brake squeal is caused by vibration between the brake pads, rotors and calipers. Pad noise can be lessened or eliminated by installing "noise suppression shims" (thin self-adhesive strips) on the backs of the pads, or applying "noise suppression compound" on the backs of the pads to dampen vibrations. Additional steps that can be taken to eliminate noise are to resurface the rotors and replace the pads.
Some brands of semi-metallic pads are inherently noisier than others because of the ingredients used in the manufacture of the friction material. Strange as it may sound (pardon the pun), cheaper pads are sometimes quieter than premium quality or original equipment pads. That's because the cheaper pads contain softer materials that do not wear as well. For that reason, they are not recommended. Premium quality pads should cause no noise problems when installed properly and will give you better brake performance and longer life.
Conditions that can contribute to a disc brake noise problem include glazed or worn rotors, too rough a finish on resurfaced rotors, loose brake pads, missing pad insulators, shims, springs or anti rattle clips, rusty or corroded caliper mounts, worn caliper mounts, and loose caliper mounting hardware. Drum noise may be due to loose or broken parts inside the drum.
Most experts recommend new caliper and drum hardware when the brakes are relined, a thorough inspection of the calipers and rotors for any wear or other conditions that might have an adverse affect on noise or brake performance, and resurfacing the rotors (and drums) if the surfaces are not smooth, flat and parallel.
If you hear metallic scraping noises, on the other hand, it usually means your brake linings are worn out and need to be replaced -- especially if your brake pedal feels low or if you've noticed any change in the way your vehicle brakes (it pulls to one side when braking, it requires more pedal effort, etc.).
Some brake pads have built-in "wear sensors" that produce a scraping or squealing noise when the pads become worn. In any event, noisy brakes should always be inspected to determine whether or not there's a problem. And don't delay! If the pads have worn down to the point where metal-to-metal contact is occurring, your vehicle may not be able to stop safely, and you may score the rotors or drums to the point where they have to be replaced.
Question:
How can I tell if a rotor or drum really needs to be replaced?
Answer:
A rotor must be replaced if it is at or below the minimum thickness specification or discard thickness stamped on the rotor (this same information can also be found in brake service manuals). Replacement is also necessary if a rotor cannot be resurfaced without exceeding the minimum thickness specification or the discard thickness specification. Replacement is also required if the rotor is cracked or damaged. Replacement may be recommended if a rotor has hard spots, is warped, or has been previously resurfaced for a warped condition.
A drum must be replaced if it is at or beyond the maximum inside diameter specification or discard diameter stamped on the drum. Replacement is also necessary if a drum cannot be resurfaced without exceeding the maximum diameter specification or discard diameter specification. Replacement is also required if a drum is cracked, damaged, bell mouthed or too far out of round for resurfacing.
Question:
Is it always necessary to resurface the rotors and drums when the brakes are relined or to rebuild or replace the disc brake calipers and drum brake wheel cylinders?
Answer:
No. The rule here is resurface when necessary, don't resurface when it isn't necessary. If the rotors and drums are in relatively good condition (smooth and flat with no deep scoring, cracks, distortion or other damage), they do not have to be resurfaced. Resurfacing unnecessarily reduces the thickness of these parts, which in turn shortens their remaining service life.
According to the uniform inspection guidelines developed by the Motorist Assurance Program (MAP), "friction material replacement alone does not warrant rotor reconditioning." Whether or not the rotors or drums need resurfacing or replacing depends entirely on their condition at the time the brakes are relined.
Even so, many mechanics prefer to resurface rotors and drums when relining the brakes to restore the friction surfaces to "like-new" condition and to minimize any chance of brake squeal.
The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.
A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.
The hydraulic and friction components are housed in a caliper assembly. The caliper assembly straddles the outside diameter of the hub and rotor assembly. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.
Most late-model cars use the moving caliper design. This design uses a single hydraulic piston and a caliper that can float or slide during application. Floating designs 'float' or move on pins or bolts. In sliding designs, the caliper slides sideways on machined surfaces. Both designs work in basically the same way.
The single-piston caliper assembly is constructed from a single casting that contains one large piston bore in the inboard section of the casting. Inboard refers to the side of the casting nearest the center line of the car when the caliper is mounted. A fluid inlet hole and bleeder valve hole are machined into the inboard section of the caliper and connect directly to the piston bore.
The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.
The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.
The caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper. Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Since the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.
The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.
As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.
Sliding calipers are made to slide back and forth on the steering knuckle support to which it is mounted. There is a V shaped surface, sometimes called a rail, on the caliper that matches a similar surface on the steering knuckle support. These two mating surfaces allow the caliper to slide in and out. The internal components of the caliper are the same as those previously described.
The stationary or fixed caliper has a hydraulic piston on each side of the rotor. Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.
Question:
My brakes are squealing. Does that mean I need a brake job?
Answer:
Not necessarily. A certain amount of brake noise is considered "normal" these days because of the harder semi-metallic brake pads that are used in most front-wheel drive cars and minivans. This type of noise does not affect braking performance and does not indicate a brake problem. However, if the noise is objectionable, there are ways to eliminate it.
Brake squeal is caused by vibration between the brake pads, rotors and calipers. Pad noise can be lessened or eliminated by installing "noise suppression shims" (thin self-adhesive strips) on the backs of the pads, or applying "noise suppression compound" on the backs of the pads to dampen vibrations. Additional steps that can be taken to eliminate noise are to resurface the rotors and replace the pads.
Some brands of semi-metallic pads are inherently noisier than others because of the ingredients used in the manufacture of the friction material. Strange as it may sound (pardon the pun), cheaper pads are sometimes quieter than premium quality or original equipment pads. That's because the cheaper pads contain softer materials that do not wear as well. For that reason, they are not recommended. Premium quality pads should cause no noise problems when installed properly and will give you better brake performance and longer life.
Conditions that can contribute to a disc brake noise problem include glazed or worn rotors, too rough a finish on resurfaced rotors, loose brake pads, missing pad insulators, shims, springs or anti rattle clips, rusty or corroded caliper mounts, worn caliper mounts, and loose caliper mounting hardware. Drum noise may be due to loose or broken parts inside the drum.
Most experts recommend new caliper and drum hardware when the brakes are relined, a thorough inspection of the calipers and rotors for any wear or other conditions that might have an adverse affect on noise or brake performance, and resurfacing the rotors (and drums) if the surfaces are not smooth, flat and parallel.
If you hear metallic scraping noises, on the other hand, it usually means your brake linings are worn out and need to be replaced -- especially if your brake pedal feels low or if you've noticed any change in the way your vehicle brakes (it pulls to one side when braking, it requires more pedal effort, etc.).
Some brake pads have built-in "wear sensors" that produce a scraping or squealing noise when the pads become worn. In any event, noisy brakes should always be inspected to determine whether or not there's a problem. And don't delay! If the pads have worn down to the point where metal-to-metal contact is occurring, your vehicle may not be able to stop safely, and you may score the rotors or drums to the point where they have to be replaced.
Question:
How can I tell if a rotor or drum really needs to be replaced?
Answer:
A rotor must be replaced if it is at or below the minimum thickness specification or discard thickness stamped on the rotor (this same information can also be found in brake service manuals). Replacement is also necessary if a rotor cannot be resurfaced without exceeding the minimum thickness specification or the discard thickness specification. Replacement is also required if the rotor is cracked or damaged. Replacement may be recommended if a rotor has hard spots, is warped, or has been previously resurfaced for a warped condition.
A drum must be replaced if it is at or beyond the maximum inside diameter specification or discard diameter stamped on the drum. Replacement is also necessary if a drum cannot be resurfaced without exceeding the maximum diameter specification or discard diameter specification. Replacement is also required if a drum is cracked, damaged, bell mouthed or too far out of round for resurfacing.
Question:
Is it always necessary to resurface the rotors and drums when the brakes are relined or to rebuild or replace the disc brake calipers and drum brake wheel cylinders?
Answer:
No. The rule here is resurface when necessary, don't resurface when it isn't necessary. If the rotors and drums are in relatively good condition (smooth and flat with no deep scoring, cracks, distortion or other damage), they do not have to be resurfaced. Resurfacing unnecessarily reduces the thickness of these parts, which in turn shortens their remaining service life.
According to the uniform inspection guidelines developed by the Motorist Assurance Program (MAP), "friction material replacement alone does not warrant rotor reconditioning." Whether or not the rotors or drums need resurfacing or replacing depends entirely on their condition at the time the brakes are relined.
Even so, many mechanics prefer to resurface rotors and drums when relining the brakes to restore the friction surfaces to "like-new" condition and to minimize any chance of brake squeal.
Repair Guide: Brake System Basics
Braking action begins when the driver pushes on the brake pedal. The brake pedal is a lever, pivoted at one end, with the master cylinder push rod attached to the pedal near the pivot point. With this lever arrangement, the force applied to the master cylinder piston through the push rod is multiplied several times over the force applied at the brake pedal.
The bracket is mounted to the inside of the engine compartment cowl or firewall. The master cylinder push rod that connects the pedal linkage to the master cylinder goes through a hole in the firewall.
The master cylinder is mounted on the opposite side of the firewall in the engine compartment. If the car has manual brakes, the cylinder will be mounted directly to the firewall. If a power booster is used, it will be mounted to the firewall and the cylinder is mounted to the booster.
Question:
How do I know when my car really needs a brake job?
Answer:
You need a "brake job" when your brake linings are worn down to the minimum acceptable thickness specified by the vehicle manufacturer or the applicable state agency in areas that set their own requirements. The only way to determine if new linings are required, therefore, is to inspect the brakes.
You may also need a brake job if you're having brake problems such as grabbing, pulling, low or soft pedal, pedal vibration, noise, etc., or if some component in your brake system has failed. But if the problem is isolated to only one component, there's no need to replace other parts that are still in perfectly good working order.
There is no specific mileage interval at which the brakes need to be relined because brake wear varies depending on how the vehicle is driven, the braking habits of the driver, the weight of the vehicle, the design of the brake system and a dozen other variables. A set of brake linings that last 70,000 miles or more on a car driven mostly on the highway may last only 30,000 or 40,000 miles on the same vehicle that is driven mostly in stop-and-go city traffic.
As a rule, the front brakes wear out before the ones on the rear because the front brakes handle a higher percentage of the braking load -- especially in front-wheel drive cars and minivans. So many service facilities advertise $59.95 brake job "specials" that replace the linings on the front brakes only. Doing the front brakes only is okay and can save you money as long as the rear brakes are in good condition. But if the rear brakes need attention, they should be relined too.
One of the problems with the brake specials you see advertised in the newspaper is that the price is very misleading. A person typically goes in expecting to spend $59.95 for a brake job, but usually ends up spending considerably more because the brakes need more than the minimum amount of work to restore them to like-new condition. The price of a brake job depends entirely on the work that needs to be performed. So any advertised special is not a firm price, but only an estimate of the least amount of money it might cost you to get your brakes fixed. A price should not be quoted until after the brakes have been inspected. Then and only then can an accurate determination be made of the parts that actually need to be replaced.
Question:
What parts are generally replaced during a brake job, and why?
Answer:
A traditional brake job (if there is such a thing) usually means replacing the front disc brake pads, resurfacing the rotors, replacing the rear drum brake shoes, resurfacing the drums, bleeding the brake lines (replacing the old brake fluid with new and getting all the air out of the lines), inspecting the system for leaks or other problems that might require additional repairs, and checking and adjusting the parking brake.
Some brake jobs may also include new hardware for the drums (recommended), and rebuilding or replacing the wheel cylinders and calipers (also recommended). But because of the added expense, these items may not be included in the package price or may only be done if the brake system really needs them (as opposed to doing them for preventative maintenance).
Hardware includes things like return springs, hold down springs and other clips and retainers found in drum brakes. It may also include bushings, pins and clips on disc brake calipers. Springs lose tension with age and exposure to heat. Most experts recommend replacing the hardware when relining drum brakes to restore proper brake action. If weak springs are reused, the shoes may drag against the drums causing accelerated shoe wear, a pull to one side, brake overheating and possible drum damage. Other hardware that is badly corroded or faulty (such as the self-adjusters) may prevent the shoes from maintaining the correct drum clearance (which increases the distance the brake pedal must travel as the shoes wear), or the parking brake from functioning properly.
It's important to note that not all replacement linings are the same. There are usually several grades of quality in pads and shoes (good, better and best). The difference is in the ingredients that are used to manufacture the pads and shoes. The less expensive ones may cost less initially and save you a few dollars on your total bill, but you may not be happy with the way they wear and perform. All brake linings must meet minimum government safety standards. Even so, the cheaper grade of pads and shoes do not last as many miles as the premium grade of replacement linings, nor do they brake as effectively. They usually have a greater tendency to fade at high temperature and may increase the vehicle's stopping distance somewhat. Noise may also be a problem with cheap linings. The best performance and value for your money, therefore, is with the best or premium grade. Choose these when the brakes are relined.
The bracket is mounted to the inside of the engine compartment cowl or firewall. The master cylinder push rod that connects the pedal linkage to the master cylinder goes through a hole in the firewall.
The master cylinder is mounted on the opposite side of the firewall in the engine compartment. If the car has manual brakes, the cylinder will be mounted directly to the firewall. If a power booster is used, it will be mounted to the firewall and the cylinder is mounted to the booster.
Question:
How do I know when my car really needs a brake job?
Answer:
You need a "brake job" when your brake linings are worn down to the minimum acceptable thickness specified by the vehicle manufacturer or the applicable state agency in areas that set their own requirements. The only way to determine if new linings are required, therefore, is to inspect the brakes.
You may also need a brake job if you're having brake problems such as grabbing, pulling, low or soft pedal, pedal vibration, noise, etc., or if some component in your brake system has failed. But if the problem is isolated to only one component, there's no need to replace other parts that are still in perfectly good working order.
There is no specific mileage interval at which the brakes need to be relined because brake wear varies depending on how the vehicle is driven, the braking habits of the driver, the weight of the vehicle, the design of the brake system and a dozen other variables. A set of brake linings that last 70,000 miles or more on a car driven mostly on the highway may last only 30,000 or 40,000 miles on the same vehicle that is driven mostly in stop-and-go city traffic.
As a rule, the front brakes wear out before the ones on the rear because the front brakes handle a higher percentage of the braking load -- especially in front-wheel drive cars and minivans. So many service facilities advertise $59.95 brake job "specials" that replace the linings on the front brakes only. Doing the front brakes only is okay and can save you money as long as the rear brakes are in good condition. But if the rear brakes need attention, they should be relined too.
One of the problems with the brake specials you see advertised in the newspaper is that the price is very misleading. A person typically goes in expecting to spend $59.95 for a brake job, but usually ends up spending considerably more because the brakes need more than the minimum amount of work to restore them to like-new condition. The price of a brake job depends entirely on the work that needs to be performed. So any advertised special is not a firm price, but only an estimate of the least amount of money it might cost you to get your brakes fixed. A price should not be quoted until after the brakes have been inspected. Then and only then can an accurate determination be made of the parts that actually need to be replaced.
Question:
What parts are generally replaced during a brake job, and why?
Answer:
A traditional brake job (if there is such a thing) usually means replacing the front disc brake pads, resurfacing the rotors, replacing the rear drum brake shoes, resurfacing the drums, bleeding the brake lines (replacing the old brake fluid with new and getting all the air out of the lines), inspecting the system for leaks or other problems that might require additional repairs, and checking and adjusting the parking brake.
Some brake jobs may also include new hardware for the drums (recommended), and rebuilding or replacing the wheel cylinders and calipers (also recommended). But because of the added expense, these items may not be included in the package price or may only be done if the brake system really needs them (as opposed to doing them for preventative maintenance).
Hardware includes things like return springs, hold down springs and other clips and retainers found in drum brakes. It may also include bushings, pins and clips on disc brake calipers. Springs lose tension with age and exposure to heat. Most experts recommend replacing the hardware when relining drum brakes to restore proper brake action. If weak springs are reused, the shoes may drag against the drums causing accelerated shoe wear, a pull to one side, brake overheating and possible drum damage. Other hardware that is badly corroded or faulty (such as the self-adjusters) may prevent the shoes from maintaining the correct drum clearance (which increases the distance the brake pedal must travel as the shoes wear), or the parking brake from functioning properly.
It's important to note that not all replacement linings are the same. There are usually several grades of quality in pads and shoes (good, better and best). The difference is in the ingredients that are used to manufacture the pads and shoes. The less expensive ones may cost less initially and save you a few dollars on your total bill, but you may not be happy with the way they wear and perform. All brake linings must meet minimum government safety standards. Even so, the cheaper grade of pads and shoes do not last as many miles as the premium grade of replacement linings, nor do they brake as effectively. They usually have a greater tendency to fade at high temperature and may increase the vehicle's stopping distance somewhat. Noise may also be a problem with cheap linings. The best performance and value for your money, therefore, is with the best or premium grade. Choose these when the brakes are relined.
Repair Guide: Brake Rotors and Pads
Disc brakes are used on the front wheels of most cars and on all four wheels on many cars. A disc rotor is attached to the wheel hub and rotates with the tire and wheel. When the driver applies the brakes, hydraulic pressure from the master cylinder is used to push friction linings against the rotor to stop it.
The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.
A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.
The hydraulic and friction components are housed in a caliper assembly. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.
The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.
The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.
As already mentioned, the caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper.Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Because the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.
The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.
As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.
Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.
The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.
A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.
The hydraulic and friction components are housed in a caliper assembly. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.
The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.
The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.
As already mentioned, the caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper.Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Because the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.
The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.
As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.
Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.
Subscribe to:
Posts (Atom)
