42 BRAKES Fig. 46 5.3 Balancing (Fig. 46) : When the output force is conveyed through the push rod to the master cylinder, the reaction force from the master cylinder comes on the rubber reaction disc and pushes it backwards. This causes the valve body to traverse back and the piston to contact the valve rubber cup, thereby closing the air valve (Y) and vacuum valve (X). In this condition both the vacuum and atmospheric valves are closed. As a result, there is a fixed pressure differential between chambers 2 & 4 and chambers 1 & 3, so the level of power assist and braking force remains constant. The vacuum brake valve always seeks balancing position whenever the brake pedal force is constant.
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