Question about GMC Sierra 1500

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Brakes problem I have a 1996 GMC Sierra 4X4. When I press the brake pedal, I hear a hissing noise. It happened just this morning when I was leaving for work. The brakes work, but are not as responsive as normal. Is it my master cyliner, or booster?

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  • Richard Scordino May 11, 2010


  • CarySchroeder May 11, 2010

    I assume you can hear this hissing sound while parked in your garage? Pop the hood, and have someone press the brake pedal while you are under the hood listening. (Not sure of your expertise, make sure you don't have loose clothing, hair, etc. that could get caught in moving parts, stay away from the belts, fans, etc.) Where does the hissing sound come from? Also, if you continue to press on the brake pedal, does the noise continue, or does it go away? What if you press on the brake immediately after killing the engine? Several minutes after killing the engine?



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Jcakey: My name is Roger and I will answer your question for you. From what you describe the problem sounds like a bad vacum brake booster. A simple way to check is to do the following. With the engine off. Press on the brake pedal 4 or 5 times. This will deplete all vacum in the booster. Next start the engine while holding your foot on the brake pedal. The pedal should drop a couple of inches. If the pedal does not drop. The problem is the brake booster. When the booster is bad you will hear a hissing sound inside of the truck when you apply the brakes..
If the master cylinder is faulty you will wave mushy brake to no brakes.Your ABS and brake failure lights will be on. Should you need further help please just ask. I will be glad to help you. Please rate this answer as this is how we know your answer was useful.Thank You for using Fix Ya. Roger

Posted on Sep 09, 2009


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1 Answer

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

The stop lights won't go out until the pedal is depressed, as there is a switch which is activated by the pedal.

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.

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I think I would be looking at the vacuum booster for the brakes having a leak. Block off the vacuum line and try the brakes again. Make sure you have lots of room to stop. If the booster is bad then the pedal will still be hard but the RPM's and hissing should go away. Hope this helps.

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Have you checked the hardware on the disc brakes all the way around, as well as checking the rear pads? Wore out hardware could cause you to require more pressure to get the pads to move, as well as low brakes on the rear. Your braking system sends more pressure to the front of the vehicle because it requires more pressure up there. Less pressure on the rear can cause this to happen. I'm willing to bet the problem from the pedal is coming from the rear brakes somewhere and not the front. The last possibility that I can think if is a problem in the ABS solonoids, they might not be opening up all the way causing you to require more pressure.

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