Repair Guide: Master Cylinder, Brake fluid, Bleeding

The master cylinder is a type of hydraulic pump operated by the driver through the pedal and push rod. The master cylinder push rod is connected to a piston inside the cylinder. There is hydraulic fluid in front of the piston.

When the pedal is depressed, the master cylinder piston is pushed forward. The fluid in the master cylinder and the entire system, being incompressible, transmits the force exerted by the master cylinder piston to all the inner surfaces of the system. At this point, only the pistons in the wheel cylinders or caliper are free to move, and because the hydraulic fluid is not compressible, the pistons move outward to force the brake shoes against the brake drums or rotors.

There are two basic advantages of using a hydraulic system to operate the brakes. First, fluid lines are easy to route from the master cylinder to each of the wheel brake units. Second, hydraulics allow us to multiply the force used to apply the brake shoes. When you use a hydraulic jack to lift a car, you are multiplying your effort using the principles of hydraulics. This multiplying effect is done using the principles of pressure and force.

Pressure can be defined as the amount of force applied to a specific area. Suppose a hydraulic pressure of 10 psi (pounds per square inch) were applied to an object with a surface area of 16 square inches. The total applied force would equal 160 pounds (10 psi pressure times an area of 16 square inches). If the same 10 PSI were applied to an object with a surface area of 2 square inches, a force of 20 pounds would be applied. The relative size of the master cylinder piston and the pistons used at the wheel brake units allow the driver's brake pedal effort to be multiplied hydraulically.

Master Cylinder Construction and Operation

The master cylinder is constructed of two main parts: a reservoir and a master cylinder body. The reservoir provides a supply of brake fluid for cylinder operation. All current reservoirs are split designs. This means they provide two separate storage areas for two separate piston assemblies. The split design allows for separating the front and rear, or one front and one rear system, from each other in case of hydraulic failure.

The reservoir may be cast as one piece with the cylinder body or it may be a separate plastic container. All reservoirs have a removable cover or caps so that brake fluid can be added to the system. A flexible rubber diaphragm at the top of the master cylinder reservoir seals the hydraulic system from possible entrance of contamination while permitting expansion or contraction of the fluid within the reservoirs without direct venting. There are two holes, or ports, at the bottom of each reservoir section. One is called the replenishing port, the other a vent port. These ports permit passage of fluid between each pressure chamber and its fluid reservoir during certain operating conditions.

The body is a long aluminum or cast iron cylinder positioned under the reservoir. Inside the cylinder are two spool-shaped pistons. The pistons are fitted with rubber seals used to prevent fluid from leaking around the pistons. One piston is called the primary, the other the secondary. Each piston provides a separate hydraulic system for the front and rear brakes or on the diagonal system between one set of front and rear brakes. Springs in the cylinder return the pistons into position after braking. Two outlet holes provide the connection for the hydraulic lines. A snap ring holds the components inside the cylinder and a boot fits around the rear of the cylinder and push rod to prevent dirt from entering the cylinder.

The operation of the primary and secondary piston is the same. We examine how one piston works when the brakes are applied. When the brake pedal is depressed, force is transferred through the push rod to the master cylinder piston, which moves forward. After the primary seal covers the replenishing port, the master cylinder chamber is closed to the reservoir so that further piston travel builds up pressure. Fluid is then forced through the outlet into the lines leading to the wheel cylinders. When the brakes are released, return springs in the drum brake mechanisms pull the shoes away from the drums. This movement pushes the wheel cylinder pistons inward, forcing fluid back through the lines to the master cylinder. However, the master cylinder piston returns to the released position faster than fluid can fill the chamber, thus creating a momentary vacuum. To compensate for the vacuum, fluid flows from the reservoir, through the vent port, through the vent holes in the piston, and around the primary seal.

The dual master cylinder operates in the same manner as the single unit just described, except that it provides two independent systems, one for the front brakes and one for the rear, or one for each diagonal set of front and rear brakes. Under normal conditions, when the brakes are applied, the primary piston moves forward. At the same time, a combination of hydraulic pressure and the force of the primary piston spring moves the secondary piston forward. When the pistons have moved forward so that their primary seals cover the replenishing ports, hydraulic pressure is built up and transmitted to the front and the rear wheels.

In case of a hydraulic failure in the rear brake system, the primary piston will move forward when the brakes are applied, but will not build up hydraulic pressure. Only a small force is transferred to the secondary piston through the primary piston spring until the piston extension screw comes in contact with the secondary piston. Then, push rod force is transmitted directly to the secondary piston and enough pressure is built up to operate the front brakes.

If there is a hydraulic failure in the front brake system, both pistons will move forward when the brakes are applied, just like normal. Due to the front system failure, however, there is nothing to resist secondary piston travel except the secondary piston spring. This permits the primary piston to build up only negligible pressure until the secondary piston bottoms in the cylinder bore. Then, enough hydraulic pressure will be built up to operate the rear brakes.

Brake Master Cylinder Fluids

The brake system uses hydraulic power generated by a master cylinder to activate the four wheel brake assemblies. A fluid reservoir is located on top of the master cylinder. The fluid level in the brake master cylinder must be checked regularly at intervals specified by the manufacturer.

Brake fluid is a specially formulated liquid that must meet Society of Automotive Engineers and federal standards. These specifications list the necessary qualities of a brake fluid. The following are the most important:

• Must flow freely at low and high temperatures.
  
• Must have a high boiling point (over 400F).
  
• Must not deteriorate metal or rubber brake parts.
  
• Must lubricate metal and rubber parts.
  
• Must be able to absorb moisture that enters the hydraulic system.
  

Brake fluid is rated by the Department of Transportation (DOT). The brake fluid is then assigned a number. The common ratings are DOT 3, DOT 4, and DOT 5. The higher the number, the higher the fluid's boiling point. The DOT rating is found on the can of brake fluid. The shop and owner's manual specify what rating is correct for the car. Do not use a brake with a lower DOT rating than specified by the manufacturer. The lower rated fluid could boil and cause a loss of brake effectiveness.

Most brake fluid is glycol based. The word 'glycol' is usually not shown on the front of the container. The word 'silicone' is shown next to the name of the brake fluid if it has a silicone base. Always use the correct type of fluid specified in the shop or owner's manual. Do not mix the types of fluids. If you use the incorrect type of fluid you might cause a loss of brake efficiency.

WARNING: Brake fluid must always be stored in clean, dry containers. Brake fluid is hygroscopic; that is, it will attract moisture and must be kept away from dampness in a tightly sealed container. When water enters brake fluid, it causes its boiling point to lower. Fluid should be protected from contamination, especially oil, grease, or other petroleum products. Never reuse brake fluid.

CAUTION: Never use gasoline, kerosene, motor oil, transmission fluid, or any fluid containing mineral oil to clean brake system components. These fluids will cause the rubber cups and seals in the master or caliper units to soften, swell, and distort, resulting in brake system failure.

Under hood Maintenance: Brake Fluid

Question: My brake pedal slowly sinks to the floor when I hold my foot on it. What's wrong?

Answer: You either have a fluid leak in your brake system or your master cylinder is defective. Either way, your brakes need immediate attention.

If the brake warning light is on, you most likely have a fluid leak. Your vehicle may not be safe to drive in this condition! You should have the brakes inspected as soon as possible to determine where the fluid is leaking (usually a hose, brake line, brake caliper or wheel cylinder) so the necessary repairs can be made.

If the brake warning light is not on, it does not necessarily mean you do not have a leak. The warning light only comes on when there's been enough fluid loss to create a pressure differential between the two sides of the hydraulic system that actually apply the brakes.

The brake system is divided into two hydraulic circuits. On most rear-wheel drive vehicles, it is divided so one circuit applies the front brakes and the other applies the rear brakes. On front-wheel drive cars and minivans, the system is usually split diagonally. One circuit works the right front and left rear brake, and the other works the left front and right rear brake. This is done for safety purposes so if one circuit loses all its brake fluid and fails, the vehicle will still have one remaining circuit to apply two wheel brakes.

A quick way to check for leaks in either circuit is to simply check the fluid level in the master cylinder reservoir. The reservoir is divided into two chambers (one for each brake circuit). If one chamber is unusually low or empty, there's a leak somewhere in that circuit. The brakes should then be inspected to check for fluid leaks. Wet spots around hose or line connections, or fluid leaking from a disc brake caliper or drum wheel cylinder would indicate a serious problem that needs immediate attention.

If the brake warning light is not on and there are no apparent leaks, then the master cylinder may be worn or leaking internally allowing the pedal to slowly sink when pressure is applied to it. This type of condition will be most noticeable when holding constant pressure against the brake pedal at a stop light. If the pedal sinks or requires pumping to keep the car from creeping ahead, the master cylinder needs to be replaced.

On some vehicles with rear-wheel antilock brake systems (ABS), it's also possible that a leak in the ABS unit may cause a similar sinking pedal condition.

Question:

My brake pedal is low when I step on it, but it comes up when I pump the brakes. Do I need new brakes?
Answer:

A low brake pedal that has to be pumped repeatedly to bring a vehicle to a stop may be due to a low fluid level, drum brakes that need adjustment or air in the lines. It usually has nothing to do with the condition of the brakes and certainly isn't grounds for a brake job.
If the pedal feels "soft" or "spongy" instead of firm, there's probably air in the system. This will require "bleeding the brakes" to remove air from the lines, calipers and wheel cylinders.

The first thing that should be checked is the fluid level in the master cylinder reservoir. If the level is low, there's a leak somewhere in the hydraulic system that must be found and repaired. Adding fluid will only cure the symptom, not the cause, and sooner or later the level will be low again creating a dangerous situation. So check for leaks around the master cylinder, wheel cylinders, brake calipers, rubber brake hoses and steel brake lines.

If the fluid level is okay, the adjustment of the rear brakes should be checked next (assuming the vehicle has drum brakes in the rear -- if it has drums all the way around, check the front drums first, then the rear). The shoes should be close enough to the drums to produce just a hint of drag when the wheels are rotated by hand. An excess of slack probably means the self-adjusters are either frozen or fully extended.

If adjusting the drum brakes fails to eliminate the low pedal, the wheel and drum will have to be removed so the adjusters can be freed up or replaced, and/or so the worn brake shoes can be replaced.

If the vehicle has rear disc brakes, the adjusting mechanism in the rear caliper pistons that maintain the correct pad-to-rotor clearance may be corroded, frozen or worn out. In most cases, the piston assemblies cannot be rebuilt and must be replaced.

If the fluid reservoir is full and the brakes are properly adjusted, but the pedal is low (or feels spongy), there is probably air in the brake lines. Air is compressible, so every time you step on the pedal, the bubbles collapse instead of transferring pressure to the brakes. The cure here is to bleed the brake lines following the factory recommended sequence.

Brakes are usually bled in a specified sequence (always refer to a shop manual for the exact procedure for your vehicle). Usually the rear brakes are bled first, then the ones up front on most rear-wheel drive cars and trucks. But on front-wheel drive cars and minivans, the hydraulic system is split diagonally so the brakes are bled in opposite pairs (right rear and left front, then left rear and right front). Following the proper sequence is important so air doesn't remain trapped in the lines.

On late model GM and Ford cars with quick take-up master cylinders, the quick take-up valve takes about 15 seconds to reseat after the brake pedal has been depressed. If the pedal is pumped too quickly while manually bleeding the system, you may never get the pedal to firm up. Most professionals use pressure bleeding equipment to bleed the brakes because it is faster and easier.