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No brake pedal when engine is running

Tiny
KRWALKER
  • MEMBER
  • 1994 CHEVROLET VAN
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
After repeated bleeding I appear to have a solid pedal when the engine is not running but when I start the engine the pedal goes directly to the floor with very little pressure. There are no brake lights on and the lights on the dash go off as usual when the key is turned on.
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Thursday, May 25th, 2017 AT 3:33 PM

5 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
You probably have air in abs unit go to a mechanic with a scanner so he can open the valves and bleed the system this one takes a specialized bleeding from abs to get it done right see pics if you want to try bleeding manually
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Thursday, May 25th, 2017 AT 3:54 PM
Tiny
JASON IBBY
  • MEMBER
  • 4 POSTS
I have a 94 k2500 suburban with the 454. Hydro boost brake system I’ve never had any issues with the brakes since I’ve had it. Randomly the pedal got soft and went all the way to the floor. I pulled over and looked for leaks but found none and master was still full. I replaced master cylinder and hydro boost and still have no pressure on the pedal when the engine is running.
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Saturday, December 14th, 2019 AT 7:03 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
When you replaced the hydroboost did it come with an accumulator? I assume it did but if this has an internal leak it will cause no pedal. Then when you replaced the master cylinder as well, did you flush the brake lines? Contamination is the other major cause of this issue.
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Saturday, December 14th, 2019 AT 3:28 PM
Tiny
JASON IBBY
  • MEMBER
  • 4 POSTS
Yeah, came with accumulator. Only thing I had to swap was the push rod and spring. Would a weak power steering pump cause a soft pedal?
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Sunday, December 15th, 2019 AT 12:08 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
I weak power steering pump would cause a hard pedal or little assistance. It relies on pressure from the power steering fluid to operate the master cylinder. Contaminated power steering fluid would be the next logical step. However, if this is contaminated it may have already effected the new hydro boost unit. I would flush the system and see what happens.
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Sunday, December 15th, 2019 AT 4:56 PM

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