As the pistons and primary seals move past each com­ pensating port, the fluid is displaced from the master cylin­ der outlets to the wheel cylinders and pressure in each of the hydraulic circuits increases. Under normal conditions the fluid pressure developed in the primary rear brake cir­ cuit (between the secondary and primary pistons) supplies the force required to operate the secondary (front brake) piston. The fluid pipes to the front and rear brakes are separated completely and both systems operate simul­ taneously with rear circuit hydraulic pressure being regul­ ated by the proportioning valve

Rear Brake (Primary Circuit) Failure

As the brake pedal is depressed the primary piston and spring moves forward. Since there is no hydraulic pressure in the primary circuit the primary spring will begin to move the secondary piston. The hydraulic pressure in the second­ ary, front circuit, will cause the primary spring to compress and allow the primary piston to contact the secondary

5 - 3A - 2

piston and provide a mechanical actuation of the second­ ary piston. The mechanical actuation of the secondary piston will pressurize the secondary circuit and apply the front brakes only.

The unbalance of the hydraulic pressure will activate the spool valve and light the brake failure warning lamp. An increase in pedal travel and effort will also be experienced.

Front Brake (Secondary Circuit) Failure

As the brake pedal is depressed the primary piston and spring will move forward and cause the secondary piston to bottom in the bore due to lack of hydraulic pressure. Once the secondary piston has bottomed, pressure is developed in the primary circuit and operates the rear brakes only.

The unbalance of hydraulic pressure will activate the spool valve and light the brake failure warning lamp. The movement of the spool valve will also pull back the proportioning valve poppet valve. This allows the

RESERVOIR CAP

• IMPORTANT: These seals MUST BE ASSEMBLED in the direction and position shown.

• SECONDARY SEAL

Fig. I-Cross sectional view of PBR tandem brake master cylinder

REV. OCT. '78