Hear air when I step on brakes............


Old 02-23-07, 11:56 AM
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Hear air when I step on brakes............

Have a 87 full size blazer, 305ci, 4x4. Last week my brakes started going to the floor. No leaks and I wasn't losing brake fluid. Someone told me to replace the master cylinder, I did that, pedal really isn't that hard. Everytime I step on the brakes, I hear a burst of air under the dashboard where the brake pedal is. It sounds like someone is letting out a big exhale everytime I hit the brakes. Can't remember if this sound was there before or not. Should I be hearing a sound like that?
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Old 02-23-07, 01:03 PM
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sounds like your brake power booster, not the master. Inspect ALL vacuum lines as well. A damaged vacuum line will cause this too.
Old 02-23-07, 04:45 PM
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Which vaccum lines should I check, and where are they?
Old 02-23-07, 04:47 PM
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Does "sound" like a booster.........but I have a question there........Normally, when a power booster leaks air and fails, you're left with a high hard brake pedal that doesn't stop the car very well.............Did the old master cyl dump a lot of brake fluid into the booster????
Old 02-23-07, 05:06 PM
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big hose

There should be a big black hose with a check valve going to the booster. It sounds like booster or check valve are shot.
Old 02-23-07, 05:27 PM
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actually I don;t think there is anything wrong with the booster or the check valve.

When a booster is activated, air is allowed to enter the back side of the booster and the front side (with the vacuum) pulls the diaphram forward as the brakes are applied.

Air must be allowed to enter the back side of the booster or it will not allow the vacuum side to pull the diaphram and "assist" as it should.

I suspect it is one of those "it was always there but simply was ignored" noises.

Now if the sound continues as the brakes are held rather than a one time "whoosh", then I can go along with a damaged diaphram in the booster.

If this were the case though, the booster should be offering no assist and the pedal would be high and hard.

as a test, everybody needs to go out and start their car, be sure the radio and blower are off and anything else that makes noise. Now step on the brakes and listen well.

oh, and by the way; the not so good pedal; either there is air entrapped in the brake lines or the master cylinder or you rear brakes need adjusting or both.
Old 02-24-07, 05:15 AM
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Yes, that is what I get , that one whoosh of air sound under the dashboard everytime I step on the brake. If I hold pressure on the brakes, I hear no air sound. The old master cylinder never leaked any fluid in the booster, brakes would go to the floor, cylinder would stay full. All the air was out of the master cylinder and it was full of fluid when I put it on. I was also told that if the booster was shot, I would have a very hard pedal, thats why I went with the master cylinder. I will double check everything again today and let you know. Thanks.......

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