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Originally Posted by BoxsterSenior
Thanks for the reply LB.
I checked the rigid vacuum line and grommet on the front side of the booster…..does not appear to be leaking. Further checking indicates air sucking from the bellows area (between the booster and the firewall). Strange! Would this verify the booster diaphragm has failed?
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Unfotunately, it probably does. There is an internal check valve, and this may be the problem, not the diaphram, but since it isn't replacable separately the issue is moot.
Diagnosing a bad brake booster is pretty easy, and there are 3 fairly simple ways to check.
First turn off the engine and press down on the brake pedal several times until it is difficult to push. Then, run the engine for a few minutes. While the engine is running press on the brake pedal and it should operate normally. If the pedal remains stiff, the brake booster isn't working properly or at all. Now, shut off the engine and let the car sit 10 min. or so. After 10 min., press on the brake pedal and if the pedal is stiff or can not be pumped 3 times with boost then the system is leaking vacuum.
Another way of checking the booster is with a vacuum pump like a MityVac. The vacuum pump can be used to evacuate the booster and determine if the check valve is the problem. You can also measure the leak down rate with it.
You can also check it manually by hooking a piece of hose to the inlet and blowing into it. (Do not use compressed air as it can damage the booster). When blowing into the booster it should not leak.