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Old September 4th, 2011, 06:36
1990Pioneer 1990Pioneer is offline
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Hissing sound when the brake pedal is pressed?

Hello all, Ive noticed that when I step on the brake pedal it hisses, sounds like an air leak? It makes this sound when I step down just a bit, then it will stop if I step down harder. then if pressed down all the way, it will start hissing again. Any ideas, thanks again!
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Old September 4th, 2011, 06:38
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Re: Hissing sound when the brake pedal is pressed?

quite probably a ruptured diaphragm in the brake booster
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Old September 4th, 2011, 06:56
1990Pioneer 1990Pioneer is offline
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Re: Hissing sound when the brake pedal is pressed?

Thanks Shorty, I will get in there this morning and have a closer look. Is that a costly/PITA repair?
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Old September 4th, 2011, 07:06
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Re: Hissing sound when the brake pedal is pressed?

hundred bucks (give-or-take) for a reman. unit at the local parts house.

I'd check carefully for brake fluid in the booster before replacing just the booster, though. If the master cylinder has a leak in the back of it, you will kill the new one again in short order.
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Old September 4th, 2011, 07:10
1990Pioneer 1990Pioneer is offline
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Re: Hissing sound when the brake pedal is pressed?

Thank you, I appreciate the tips!
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Old September 4th, 2011, 11:38
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Re: Hissing sound when the brake pedal is pressed?

Could be normal. The way power brakes work is that there is a rubber diaphragm in the big metal drum behind the master cylinder. In normal driving operation (not braking), vacuum pressure is applied to both sides of the diaphragm. When you step on the brakes, the cabin side of the diaphragm vents to atmosphere, which causes that side to equalize to normal air pressure. Meanwhile the engine side is still under vacuum, so the diaphragm gets pulled that way. That is the power brake assist--a big diaphragm under engine vacuum gets pulled towards the engine, which helps you to push the brake rod through the master cylinder.

Sometimes you will hear the whoosh noise when you step on the pedal, which is air rushing into the cabin side of the diaphragm. If you hear a constant noise when the pedal is down, then you might have a tear or hole in the diaphragm, allowing the engine vacuum to leak into the cabin side.

Do your brakes work alright or are they really hard? Do you hear the noise always or just when pressing the pedal?
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Old September 4th, 2011, 11:47
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Re: Hissing sound when the brake pedal is pressed?

Quote:
Do your brakes work alright or are they really hard? Do you hear the noise always or just when pressing the pedal?
.

I agree. If you have a leaking diagram then you will have a much heavier pedal pressure to stop. If it is stopping fine without having to stand on it then I is probably fine.
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Old September 4th, 2011, 12:09
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Re: Hissing sound when the brake pedal is pressed?

Sometimes the foam silencer inside the booster's pushrod hole (which is where it vents from when the pedal is depressed) wears out or degrades allowing the noise to become louder. It is not a replacement part, nor does it affect the operation of the booster. If you can tolerate the noise and the booster operates normally, leave it be. If there is any sign of improper operation, change it.
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Old September 4th, 2011, 14:25
1990Pioneer 1990Pioneer is offline
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Re: Hissing sound when the brake pedal is pressed?

It only makes the sound when I press the pedal. The brakes work just fine, I dont have to put much effort into stopping. When I push down the pedal it will make the sound constantly until I press it to the floor then it stops.
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Old September 4th, 2011, 15:17
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Re: Hissing sound when the brake pedal is pressed?

I have found that to be an early warning sign that the booster is on its last legs. Sometimes it takes a good while for them to go completely bad (even a year or two), other times if the brake fluid is leaking in the booster area, it fails much faster.

Check for leaking, missing brake fluid in the MC, that never shows up anywhere!!!! Keep a close eye on it, until the brake performance is an issue.

If the vacuum leak gets bad enough it will cause a high idle at idle with the brakes applied.
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