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.