Brake pedal goes to floor

1998 CHEVROLET SILVERADO

190,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC

HURLER61

MEMBER
2 POSTS

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

Dec 9, 2016 at 3:34 AM

HMAC300

✓Experienced 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.

Feb 22, 2021 at 9:29 AM

STEVE W.

✓Experienced Mechanic
15,360 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.

Feb 22, 2021 at 9:29 AM

HURLER61