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Brakes go to floor when motor is cranked

Tiny
SLAPOUT
  • MEMBER
  • 1990 CHEVROLET TRUCK
  • 190 MILES
Replaced leaking rear wheel cylinders, shoes, and drums. After bleeding until no bubbles, and firm pedal, started engine and the pedal dropped almost to the floor. Bled more. Had small leak under reservoir, so I replaced master cylinder. Bled more. Have firm pedal, till I start the engine. Bled master, bled all 4 brakes, again. So far used 3 qt, s of fluid. Can it be the ABS hydraulic unit? Pedal is firm and does not drop until I start it up.
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Thursday, December 8th, 2011 AT 1:52 AM

2 Replies

Tiny
DRCRANKNWRENCH1
  • MECHANIC
  • 3,435 POSTS
The brake booster is bad.
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Thursday, December 8th, 2011 AT 2:00 AM
Tiny
HOMER1967
  • MECHANIC
  • 875 POSTS
You have a bypassing brake booster. You need a new one.
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Thursday, December 8th, 2011 AT 2:06 AM

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