Master Cylinder
The master cylinder is the primary unit in the brake system that converts the force of the operator's foot into fluid pressure to operate the wheel cylinders. It is normally mounted to the firewall, which allows for easy inspection and service, and is less prone to dirt and water. The master cylinder has four basic functions that are as follows:

It develops pressure, causing the wheel cylinder pistons to move towards the drum or rotor.

After all of the shoes or pads produce sufficient friction, the master cylinder assists in equalizing the pressure required for braking.

It keeps the system full of fluid as the brake linings wear.

It can maintain a slight pressure to keep contaminants (air and water) from entering the system.

In its simplest form, a master cylinder consists of a housing, a reservoir, a piston, a rubber cup, a return spring, a rubber boot, and a residual pressure check valve (fig. 7-4). There are two ports (inlet port and compensating port) drilled between the cylinder and reservoir. The description of the components of a master cylinder is as follows:

The master cylinder housing is an aluminum or iron casting having either an integral or detachable reservoir. A cylinder is machined in the housing of the master cylinder. The spring, the cups, and the metal piston move within this cylinder.

The piston is a long spoonlike member with a rubber secondary cup seal at the outer end and a rubber primary cup at the inner end, which are used to pressurize the brake system. The primary cup is held against the end of the piston by the return spring. A steel stop disc, held in the outer end of the cylinder by a retainer spring, acts as a piston stop.

Figure 7-4.- Cutaway view of a single master cylinder.

A rubber boot prevents dust, dirt, and moisture from entering the back of the master cylinder. The boot fits over the master cylinder housing and the brake pedal pushrod.

The reservoir carries a sufficient reserve of fluid to allow for expansion and contraction of brake fluid and brake lining wear. The reservoir is filled at the top and is well sealed by a removable filler cap containing a vent. Integral reservoirs are made of the same material as the cylinder. whereas detachable reservoirs are made of plastic.

The intake port or vent allows fluid to enter the rear of the cylinder, as the piston moves forward. Fluid flows out of the reservoir, through the intake port, and into the area behind the piston and cup.

The compensating port releases extra pressure when the piston returns to the released position. Fluid can flow back into the reservoir through the compensating port. The action of both ports keeps the system full of fluid.

The residual pressure check valve maintains residual fluid pressure of approximately 10 psi. This pressure prevents fluid from seeping past the cups in the wheel cylinders and also prevents air from entering the hydraulic passages when the brakes are released.

Older vehicles used single piston, single reservoir master cylinders that were dangerous. If a fluid leak developed (cracked brake hose, seal damage, or line rupture). a sudden loss of braking ability occurred. Modern vehicles use dual master cylinders. These master cylinders provide an additional safety feature in that should one portion of the brake system fail. the other system will allow the vehicle to maintain some braking ability.

The dual master cylinder (fig. 7-5). also called a tandem master cylinder, has two separate hydraulic

Figure 7-5.- Dual master cylinder.

pistons and two fluid reservoirs. In the dual master cylinder, the rear piston assembly is termed the primary piston and the front piston is termed the secondary piston.

In some dual master cylinders, the individual systems are designed where one master cylinder piston operates the front brake assemblies and the other operates the rear brake assemblies. This is known as a longitudinally split system (fig. 7-6). A system that has each master cylinder piston operating the brake assembly on opposite corners of the vehicle is known a diagonally split system (fig. 7-6). In either system, if there is a leak, the other master cylinder system can still provide braking action on two wheels.

When the systems are intact (no leaks), the pistons produce and supply pressure to all four of the wheel cylinders. However, if there is a pressure loss in the primary circuit of the brake system (rear section of the master cylinder), the primary piston slides forward and pushes on the secondary piston. As shown in figure 7-5, this action forces the secondary piston forward mechanically, building pressure in two of the wheel cylinder assemblies. Should the secondary circuit fail, braking for the other two wheels would still be

Figure 7-6.- Dual master cylinder braking systems.

available. The secondary piston slides completely forward in the cylinder, as shown in figure 7-5. Then the primary piston provides hydraulic pressure to the other two brake assemblies. It is very unlikely that both systems will fail at the same time.

When performing maintenance on a dual master cylinder, you may notice that the front reservoir is larger than the rear. This is a longitudinally split system. The larger reservoir is for disc brakes. The larger reservoir is necessary because as the disc pads wear, they move outward creating a larger cavity in the caliper cylinder and fluid moves from the master cylinder to fill the additional area. To allow this action to occur, the front reservoir of a longitudinally split system has no residual check valve. However, with a diagonally split system both reservoirs are the same size and the residual check valve for the rear brakes are located in the tees that split the system front to rear.