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
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
  • 13,414 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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