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 I 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.
A wheel cylinder (fig. 7-7) changes hydraulic pressure into mechanical force that pushes the brake shoes against the drums. Other than the standard wheel cylinder, there are two other types that you may come in contact with - the stepped wheel cylinder and the single-piston wheel cylinder.
The stepped wheel cylinder (fig. 7-7) is used to compensate for a faster rate of wear on the front shoe than on the rear shoe because of the self-energizing action of the brakes. This condition requires a stepped wheel cylinder with two bore sizes.
The single-piston wheel cylinder (fig. 7-7) is used when it is desired that both brake shoes be independently self-energizing, especially on the front wheels. With this design it is necessary to have two wheel cylinders, one for each shoe. Each cylinder has a single piston and is mounted on the opposite side of the brake backing plate from the other cylinder. Such an arrangement is shown in figure 7-8.
For further information on wheel cylinders, refer to "Drum Brake Assemblies" in this chapter.
Brake Lines and Hoses
Brake lines and hoses transmit fluid under pressure from the master cylinder to the wheel cylinders. The brake lines are made of double-wall steel tubing with double-lap flares on their ends. Rubber brake hoses areContinue Reading