Hydraulic Brake Booster: Operation
The master cylinder hydraulic pressure for the front brakes is assisted by a brake booster which utilizes accumulator pressure. The hydraulic pressure generated by the accumulator is introduced directly to the rear brakes. See Fig 1 .
When the pedal operating force is transmitted in turn by the operating rod, the power piston and the master cylinder piston, the regulator piston and the spool valve move to the left. When the spool valve opens line A and closes line B, the accumulator pressure is supplied to the rear brakes. This pressure is introduced to the booster chamber and provides power assistance to move the master cylinder piston to the left. See Fig 2 .
Then, when the pressure in the booster chamber increases, the pressure which is applied to the rear (left side) of the reaction disc also increases, and the reaction disc starts to change shape. The reaction disc contacts the reaction rod, and the reaction rod in turn contacts the spool valve, so that the master cylinder pressure being applied towards the left and the booster pressure being applied to the right are brought into balance by the reaction disc. This allows the power-assist hydraulic pressure generated by the pedal operating force to be kept at an equilibrium.
If a loss of accumulator pressure occurs because of a problem such as a pump motor problem, the power assist force and rear brake pressure cannot increase in response to the input from the brake pedal. Accordingly, the front brakes can be operated directly by depressing the brake pedal, but the rear brakes do not operate at all.