90 GMC Half Ton Brake Problem

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Bob - 09 Mar 2007 19:36 GMT

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

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Battleax - 09 Mar 2007 20:03 GMT

> Soft brake pedal, goes to the floor only when engine is running.
> When engine is off brake pedal up where it belongs.
[quoted text clipped - 8 lines]
>
> 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.

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Whitelightning - 09 Mar 2007 23:39 GMT

> Soft brake pedal, goes to the floor only when engine is running.
> When engine is off brake pedal up where it belongs.
[quoted text clipped - 8 lines]
>
> Bob

How did you bleed the master cylinder?
Whitelightning

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Bob - 09 Mar 2007 23:53 GMT

>Soft brake pedal, goes to the floor only when engine is running.
>When engine is off brake pedal up where it belongs.
[quoted text clipped - 8 lines]
>
>Bob

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.

http://www.classicperform.com/TechBook/BrakeTroubleshoot.htm#testbooster

Thanks to those who replied.

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Whitelightning - 10 Mar 2007 01:32 GMT

>>Soft brake pedal, goes to the floor only when engine is running.
>>When engine is off brake pedal up where it belongs.
[quoted text clipped - 17 lines]
>
> 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

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Battleax - 10 Mar 2007 01:57 GMT

> 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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