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Lose brake pressure when engine is started

Tiny
WES10
  • MEMBER
  • 1999 CHEVROLET SILVERADO
  • 117,000 MILES
Brake pressure when truck is off, but once it is started the pedal goes straight to the floor. What could be the problem? We had originally thought it was the mast cylinder, and bled and replaced it. We also bled the brakes after word. Then we realized there was a busted brake line and we replaced that and then re-bled the brakes and kept fluid in the master cylinder reservoir. There is brake pressure when the truck is off, but once the truck is started I lose all brake pressure and the pedal goes straight to the floor.
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Saturday, October 1st, 2011 AT 6:17 PM

3 Replies

Tiny
CADIEMAN
  • MECHANIC
  • 3,544 POSTS
Make sure you close the bleeder before releasing the pedal. Start at r/r then l/r then the fronts. Pump the brake when you open the bleeder you should see a nice stream shoot out. If you didnt bench bleed the master you will need start by bleeding the master before the wheels.
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Monday, February 22nd, 2021 AT 5:28 PM
Tiny
WES10
  • MEMBER
  • 2 POSTS
Thank you, we did exactly what you said and it worked, that was the one thing we were doing wrong, we werent holding down the brake pedal before tightening the bleeder screw. Thanks again
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Monday, February 22nd, 2021 AT 5:28 PM
Tiny
CADIEMAN
  • MECHANIC
  • 3,544 POSTS
We all learn by trial and error. Ty for using 2carpros
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Monday, February 22nd, 2021 AT 5:28 PM

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