Mike C.

I was poking around under the hood this evening trying to tack down a fuel smell and hot start reluctance and happened to notice that when the car is shut down, a distinct hissing noise is heard at the brake booster. I assume the vacuum built up in the booster is leaking down (or, more accurately 'up'). Is this normal? Has anyone noticed this before or is it a sign of a vacuum leak? It takes about 12 seconds to stop making the noise; it gradually gets fainter. It's an early 85 BTW.

bearone

sounds like a vacuum leak. is the brake pedal firm or spongy when running and after the hiss is gone?

start it again after the hiss stops, with your foot on the brake pedal. if it holds, you still have vacuum, if it goes to the floor, a leak in the diaphragm.

87951

Spidey944

The brake booster is supposed to hold vacuum almost indefinitly, the one-way check valve inline with the brake booster hose is to keep vacuum in the booster allowing you to stop in the event that the engine stops. Thus giving you a few pumps of the brake pedal before vacuum runs out. if you can hear it hiss than the booster diaphram may be leaking, or the seal is leaking that the booster vacuum hose goes into.

Mike C.

Thanks guys. A project for this weekend me thinks...