90 GMC Half Ton Brake Problem

Mar 09, 2007 5 Replies

Soft brake pedal, goes to the floor only when engine is running. When engine is off brake pedal up where it belongs.



All new rotors, pads, rear drums, shoes and master cylinder.



Stumped !!!



Anyone experienced this problem?



Tnx for any constructive replies.



Bob


If the pedal goes to the floor, at any time it's either lots of air in the system, very poorly adjusted rear drum shoes, a leak, or a bad master cylinder. Many new aftermarket master cylinders are faulty.

How did you bleed the master cylinder? Whitelightning

Ok, I think I might have answered my own question.

Looks like the Power Brake Booster as it fails Test 1 on the following page for the Power Brake Booster.

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Thanks to those who replied.

I've never seen a booster go bad and cause a low pedal. when they go bad they cause a high hard pedal

Whitelightning

Ans even if it was bad it wouldn't allow the pedal to hit the floor. I'd say huge air or a poorly rebuilt master cyl.

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