Question about 1995 Pontiac Grand Am

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Brake hisses under the dash when i step on the brake pedal

Brake pedal hisses when i step on it till i let up

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  • kbfunbeach May 21, 2009

    i would like u to know i had it tested a smoke leak detection they found it was not any vacuum hoses but did determine it was the the vacuum diaphram inside the brake power booster an internal problem thank u u were right on replace the power booster.

  • brownrinard
    brownrinard Aug 17, 2012

    okay. i am having the same problem except now when i went to start my car it started fine and sounded okay but it woud die down then shut off everytime i applied the brakes. now it wont start it just turns over....what can possibly cause that?


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This is caused by a defective leaking vacuum (that is the hiss) power brake booster diaphragm, replace the power brake booster.Use the link below for general instructions on replacing your booster, it is a diff car but the same type of part is on your car.

Posted on May 20, 2009

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Sounds like your vaccum assist has a leak, open the hood and just in front of the driver on the firewall is a big round kind of a semi ball. have someone with good hearing listen next to this while someone inside pushes the brake on and off. if you can hear a hissing sound try and locate it, it might just be a bad hose or hose connection, if its hissing and its not the hose then the vaccum assist needs replacement.
good luck

Posted on May 20, 2009

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Have your mastercyclinder checked out. This sounds like a issue with your master cylinder diaphragm letting air into the breaking system causing this problem. It is a odd problem usually only happens on the bigger vehicles that need more stopping power and the breaks get used more. But the problem you are explaining sounds like the master cylinder

Posted on May 20, 2009

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My brake pedal will not budge, and a hissing noise present when running

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A hard brake pedal is probably caused by a defective servo. The hissing noise is probably an air leak in the servo.

Switch the engine on, open the bonnet (the hood..) and see if you can locate the hissing - that'll probably be in the brake servo area.

If you don't know what a brake servo looks like here is a generic image:


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Why when I step on brake an hear hissing noise under dash?

If nobody has helped you yet.....I have 2 things that are possible; 1st is the vacumn assist booster, while it sits in the engine side of the firewall your brake pedal is connected to it, it serves to 'amplify' your pedal pressure. A vacumm leak into the cab compartment may cause the hiss and the inconsistency in the way the brakes grab. The second may be a cruise control switch the earlier ones were controlled by vacumm but I do not know when they switched to electronic controls but this would have nothing to do with the break 'grab'. Best of luck.

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...maybe have an assistant push on the peddle while you listen with a piece of hose or a stethescope to find the leak (or I suppose the power unit diaphragm could be shot/leaking--hence replacement), but since you said after bumpy road etc, one could hope for a simple dislodged vacuum hose or fixable crack.

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Hissing or scratching sound is usually related to consumed brake pads/ shoes or deformed disks.
Usually you fix it by replacing brake pads, in some case you also need to change the disk, but this is less frequent, and happens when you keep circulating with consumed brake pads for many miles.
If you have a vehicle with brake shoes instead, you need to replace and calibrate brake shoes .

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