Brake Servo

BRAKES

70-10

DESCRIPTION AND OPERATION

When the brake pedal is pressed, the front push rod in the brake servo assembly pushes the primary piston along the 
cylinder bore. This produces pressure in the primary pressure chamber which, in conjunction with the primary spring, 
overcomes the secondary spring and simultaneously moves the secondary piston along the cylinder bore. The initial 
movement of the pistons, away from the piston stops, closes the primary and secondary centre valves. Further 
movement of the pistons then pressurizes the fluid in the primary and secondary pressure chambers, and thus the 
brake circuits. The fluid in the chambers, behind the pistons, is unaffected by movement of the pistons and can flow 
unrestricted through the feed holes between the chambers and the reservoir. When the brake pedal is released, the 
primary and secondary springs push the pistons back down the bore of the cylinder. As the pistons contact the piston 
stops, the primary and secondary centre valves open, which allows fluid to circulate unrestricted between the two 
hydraulic circuits and the reservoir, through the centre valves, the chambers behind the pistons and the feed holes.

Brake Servo
The brake servo assembly provides power assistance to reduce the pedal load when braking. The assembly is 
attached to the front of the pedal box and comprises a circular housing containing a diaphragm, push rods, valve and 
filter. A push rod at the rear of the housing is connected to the brake pedal. 

The vacuum line is connected to a port in the front face of the housing. With the brake pedal released and the engine 
running, vacuum pressure is present on both sides of the diaphragm. When the brake pedal is pressed, the rear push 
rod moves forward and opens the valve to allow atmospheric pressure through the filter into the pedal side of the 
diaphragm. The pressure differential acting on the diaphragm increases the pressure being applied by the brake 
pedal, which is transmitted to the master cylinder via the forward push rod. If the servo fails, the hydraulic system will 
still function but will require greater pedal effort due to the lack of vacuum assistance.