Written by: Mike Ervin
Power brakes operate just as non-power brake systems except in the actuation of the master cylinder pistons. A vacuum diaphragm is located on the front of the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel that must put into moving the brake pedal.
The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve is placed at the point where the hose enters the diaphragm housing, so that during periods of low manifold vacuum brake assist vacuum will not be lost.
Depressing the brake pedal closes off the vacuum source an allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm, and return springs return the diaphragm and master cylinder pistons to the released position. If the vacuum fails, the brake pedal rod will butt against the end of the master cylinder actuating rod, and direct mechanical application will occur as the pedal is depressed.
The hydraulic and mechanical problems that apply to conventional brake systems also apply to power brakes, and should be checked for if the tests below do not reveal the problem.
Test for a system vacuum leak as described below:
One thing I have came across when a power brake booster
goes bad, sometimes when applying the brakes the engine rpm will go up. What in
effect is happening is, the booster is causing a giant vacuum leak, thus when you depress
the pedal it lets the engine suck air. Of course the above tests should indicate
this, but if you have noticed this when driving you may want to do these tests to check
Removal and installation: