Brake Booster Makes Hissing Sound. Repair? Replace?

71 GS 350, power disc brakes, no AC, no cruise.

I recently noticed that when I shut the car off, I can hear a hissing sound coming from under the hood, near the power brake booster.

The hissing sound continues for ~10 seconds after the car is turned off. I can't tell if it's hissing when the car is running, due to engine noise.

The car stops fine, and I haven't noticed any spongy pedal, loss of braking power, etc.

I pulled the vacuum line off the booster and plugged it, and the hissing was not present. The engine did not run any more/less smooth with the booster disconnected.

Do I have a vacuum leak coming from the brake booster?

Thanks

 

Replace the check valve and grommet. Should be able to find one in the "help" section of the auto parts store.

 

If you still have a problem...call Booster Dewey or Booster Steve at Power Brake Booster Exchange
in Portland, Oregon # 503-238-8882. They are the Booster Experts and are right 99.5% of the time.
Tell them "Buick Eric" sent you.

Buick Eric / Oregon :3gears:

 

Is this brief hissing sound causing a vacuum leak?

Thanks

 

It is probably a vacuum leak. Plug the vacuum line to the booster before the check valve, and report back if there is no hiss. You do not need to drive the car for this test. Let's just take it one step at a time.

 

No, it is just the booster bleeding off after shutdown. The checkvalve is supposed to seal and hold vacuum in the booster for emergency braking after engine dies.

 

I pulled the vacuum line off the booster and plugged it, and the hissing was not present.

Also, the engine did not run any more/less smooth with the booster disconnected from the vacuum line

When I reconnect the vacuum line, the hissing condition resumes.

Thanks

 

I've never experienced this condition on any of the GM cars I've owned.

Is this bleeding down/hissing sound common?

Are you sure it's not indicative of a small vacuum leak?

Thanks

 

It is a vacuum leak, it is not common because it's a sign that something has failed. Three places the vacuum seal can break. The check valve, the grommet that holds the check valve, and the diaphragm in the booster. It's cheap and easy to change the check valve and grommet. If that doesn't cure it, then you are looking at spending some dollars and replacing the booster.

The leak isn't significant enough to effect the running of the car, likely no more different then weather changes.

 

Can anyone recommend the best brand/best place to buy a new booster?

Are there any adjustments to be made during installation, or do you just bolt it right on?

Thanks

 

If the hiss is from the grommet, you would likely hear it with engine running, in which case it would be a running "vacuum leak". If the hiss is from the check valve, it would only be audible after shut down when the vacuum drops in the intake and the high vacuum in the booster leaks back thru into the intake. If the hiss is only when pedal is depressed, it is the booster diaphram. I normally equate a "vacuum leak" with a running engine and affecting mixture or idle quality. Tomato - tomahto.

 

There is one other seal that can break and leak not mentioned in any of the above. There is seal where the rod goes through the booster and into the back of the master. Unbolt the master from the booster; there is enough flexibility in the steel brake lines to push the master out of the way and inspect the seal down in that hole. I've had one tear and cause an audible leak. They are very easily replaced and a dime-a-dozen at your local junk yard.