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Brake pedal goes to floor

Tiny
HURLER61
  • MEMBER
  • 1998 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 190,000 MILES
My brake pedal started going to the floor. I replaced master cylinder twice. New calipers, wheel cylinders, new brake lines. I bench bleed master cylinder.
Started in rear bled and bled until pedal was firm. Please note I have done this
many many times and never had a problem. Pedal firm push down real hard pedal firm. Start truck then pedal goes to floor. Remove vacuum hose from booster and the pedal gets firm.
Thanks
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Friday, December 9th, 2016 AT 11:34 AM

4 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Have a mechanic use his scanner to bleed brakes he can command valves open so they can be bled probably air in ABS unit. You can try a power bleeder to do this sometimes successful other times not. When you disconnect vacuum booster it is just like having engine off when parked.
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Monday, February 22nd, 2021 AT 5:29 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,018 POSTS
If I am reading this right the pedal is good with the engine off, but when running it drops on its own. That sounds like a failing internal booster valve. It is allowing vacuum to draw the diaphragm down. Have seen that happen a few times.
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Monday, February 22nd, 2021 AT 5:29 PM
Tiny
HURLER61
  • MEMBER
  • 2 POSTS
Inside the ABS unit?
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Monday, February 22nd, 2021 AT 5:29 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Yes, inside the ABS unit.
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Monday, February 22nd, 2021 AT 5:29 PM

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