1996 GMC Jimmy hissing sound and hard pedal

  • 1996 GMC JIMMY
Brakes problem
1996 GMC Jimmy Four Wheel Drive Automatic 120, 00 miles

I've replaced the front pads and rotors and bled the whole system. I still have a loud hissing sound when I attempt to depress the brakes. I say attempt, as it almost takes two feet to depress the pedal. Is this a wheel sensor? How do I test to determine the problem? Thanks guys!
Do you
have the same problem?
Saturday, September 26th, 2009 AT 12:53 PM

1 Reply

Sounds like the brake booster

HARD BRAKE PEDAL: Lack of power assist may be due to low engine vacuum, a leaky vacuum hose or a defective booster. Sometimes a faulty check valve will allow vacuum to bleed out of the booster causing a hard pedal when the brakes are applied. This condition can be diagnosed by starting the engine (to build vacuum), shutting it off, waiting four or five minutes, then trying the brakes to see if there is power assist. No assist means a new check valve is needed.

A quick way to check the vacuum booster is to pump the brake pedal several times with the engine off to bleed off any vacuum that may still be in the unit. Then hold your foot on the pedal and start the engine. If the booster is working, the amount of effort required to hold the pedal should drop and the pedal itself may depress slightly. If nothing happens and the vacuum connections to the booster unit are okay, a new booster is needed (the vacuum hose should be replaced, too).

On vehicles equipped with "Hydroboost" power brakes, a hard pedal can be caused by a loose power steering pump belt, a low fluid level, leaks in the power hoses, or leaks or faulty valves in the hydroboost unit itself (the latter call for rebuilding or replacing the booster).
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Saturday, September 26th, 2009 AT 1:06 PM

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