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2007 GMC 2500HD Unit has an issue where the brake pedal will…

2007 GMC 2500HD Unit has an...
2007 GMC 2500HD
Unit has an issue where the brake pedal will work fine 95% of the time but then randomly it will go all the way to the floor and then engage the brakes. Replaced the hydroboost because it had leaking shaft seals and that didn't do anything.
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Answered in 2 minutes by:
3/26/2018
JFN1
JFN1, ASE Certified Technician
Category: GMC
Satisfied Customers: 5,628
Experience: GM Master Technician
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If there is no way to replicate this so you can diagnose it, I would take an educated guess at the master cylinder. It's the only thing that makes any sense, the piston in the master cylinder has to have an intermittent leak, allowing the pedal to drop all the way down.

Please take a moment to rate my answers using the stars at the top of the page, that's how I get compensated. Let me know if you need anything else, I am happy to follow up.

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Customer reply replied 26 days ago
I blocked off both ports on the master cylinder and the pedal was rock solid. Do you think that could still be the issue?

But you said it was hard to replicate the problem, happening only 5% of the time, yes?

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Customer reply replied 26 days ago
that's correct. I just assumed with the ports blocked off and if the cylinder was leaking it would slowly bypass.

I think it seals most of the time, and occasionally when it's depressed, it's bypassing fluid until it catches a seal at the bottom of the bore. This is how I have envisioned it based on your description.

There is no fluid loss (at least you didn't mention any). So it's not due to a caliper issue. The ABS system doesn't have the ability to cause this sort of problem. Basically speaking, it can't be anything other than a master cylinder.

JFN1
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Customer reply replied 26 days ago
Okay, I will try to swap that out. Thanks!
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