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Old 12-15-2008, 07:38 AM   #1
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Power Brake Booster, Question

I have a hissing sound (like a vacuum leak) coming from the power brake booster. It can even be heard from inside the car when sitting in the driver’s seat. The sound goes away when the brake pedal is fully depressed, and returns when the pedal is released. Is this normal, or do I have a leaking brake booster, or something else? Any information would be appreciated.

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Old 12-15-2008, 08:43 AM   #2
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Yep, almost certainly a vacuum leak.

The booster itself is a pretty dumb part, a vacuum vessel, rubber diaphram and a check valve. It's usually the internal diaphram which fails, but for the Boxster, you can't repair/replace it; you must replace the whole part - 996.355.923.00 (for 986) and it isn't cheap.

But there is also a gasket on the housing which the vac line plugs into and these too can leak so it's worth checking them b4 forking out for the whole unit.

From the way you describe it and the sound, it does sound like the a leak in the line or gasket, so check these first.

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Old 12-16-2008, 04:22 AM   #3
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Power Brake Booster

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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Old 12-16-2008, 11:30 AM   #4
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Quote:
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?

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.
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Last edited by Lil bastard; 12-16-2008 at 11:32 AM.
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Old 12-16-2008, 04:43 PM   #5
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Power Brake Booster

Again, thanks for the reply message LB.
Well, confirmed by method one and three, the brake booster is leaking. Now I know what to ask Santa Claus for.
Replacing it looks pretty straight forward with the only PITA having to bleed the brake and clutch system. Any hints or suggestions for a DIY replacement project?
Thanks!
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Old 12-16-2008, 11:38 PM   #6
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Quote:
Originally Posted by BoxsterSenior
Again, thanks for the reply message LB.
Well, confirmed by method one and three, the brake booster is leaking. Now I know what to ask Santa Claus for.
Replacing it looks pretty straight forward with the only PITA having to bleed the brake and clutch system. Any hints or suggestions for a DIY replacement project?
Thanks!
Sent you a PM

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Old 01-15-2009, 05:17 AM   #7
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I don't hear a hissing sound but with the engine running my brakes actually apply themself. Turn off the engine and the brakes release. I was told this happens when the brake booster goes bad......Has anyone heard of this, I never have before.
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