1999 Jeep Grand Cherokee Air noise

1999 JEEP GRAND CHEROKEE
100,000 MILES • 4.7L • V8 • AWD • AUTOMATIC
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GEISER
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  • 2 POSTS
After a deepfrozen night I started the car in the morning without any problems, but in the same moment, a hizzing sound (like air escaping) started. Looking for the problem I found that the noise was coming from the end of the brake pedal where that dustcap is. After starting the engine all warning lights go off and nothing shows a failure. When I step on the brake it works fine and the noise stopps. As soon as I release the brake it starts again and constantly. Turning off the engine it stopps and the brake is hard again. Turning on the engine again it starts again (without having touched anything).
When I tested the brake (pumping the pedal twice until hard, whilst staying on the brake starting the engine the brakepedal went immediately soft) and on the road itself everything works perfectly. No lamps come on but the sound of escaping air stays.

Do you know what the problem might be?
Thanks for a fast answer
Jürg
Dec 29, 2014 at 10:34 AM
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CADIEMAN
  • ASE Certified Mechanic
  • 3,544 POSTS
  • ASE Certified Mechanic
YOU WILL NEED TO REPLACE THE BRAKE BOOSTER. IT HAS A SEAL LEAKING VACUUM. THE BOOSTER IS WHAT THE MASTER CYLINDER MOUNTS TO.
Dec 29, 2014 at 10:54 AM
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