Section through brake servo assembly - brakes off (page 2 of 2)

During the initial movement of the piston, the valve, assisted by the valve spring, moves with the piston and closes the vacuum port to isolate the chambers at the rear of the diaphragms from the vacuum source. Further movement of the input push rod causes the piston to move away from the valve and open the air inlet port. This allows a restricted flow of filtered ambient air through the air inlet port, which creates a servo pressure in the chambers at the rear of the diaphragms. Force from the resultant pressure differential across the diaphragms is transmitted through the valve body to the output push rod, augmenting the pressure being applied by the brake pedal. The force produced by the diaphragms, in proportion to the input force, i.e. the boost ratio, is 5.6 : 1. The boost ratio remains constant, as the input force from the brake pedal increases, until the limit of assistance is reached when servo pressure is equal to ambient pressure.

Brakes held on When the brake pedal effort is constant, opposing pressures cause the reaction disc to extrude onto the ratio disc, which moves the piston against the valve to close the air inlet port. This prevents any further increase in servo pressure and maintains a constant output force to the master cylinder assembly.

Brakes released When the brake pedal is released, the input rod spring moves the input rod and piston rearwards within the valve body to close the air inlet port and open the vacuum port. The air from the chambers at the rear of the diaphragms is then evacuated, through the vacuum port and the chambers at the front of the diaphragms, to restore a partial vacuum in all four chambers. Simultaneously, the return spring moves the valve body, diaphragms, output rod and input rod rearwards to return them to their brakes off position.