GMT 900 Trucks General Discussion 2007 - 2013 Trucks | General Discussion

Bad master cylinder or booster?

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Old 09-22-2016 | 12:27 PM
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vmaxpro's Avatar
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Default Bad master cylinder or booster?

Brake fluid was towards the minimum line so I check for leaks and found nothing. Then I checked inside the booster and sure enough had fluid inside. Is this a common problem for nnbs? I was told Its a seal on the pushrod pulling vacuum on the MC so I just need a booster. Then I was told I need both. I can easily do the work myself but Im scared of getting air in the abs and having to tow it. Any ideas? The pedal is rock hard with no vacuum, but when running if I push pretty hard I can bottom out the Pedal. Thanks!
Old 09-22-2016 | 05:52 PM
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I pulled mc off. There is no pushrod seal. Theres an o ring on the mc to seal the vacuum. So much for that guys theory. The brake fluid is 130k old. Would old fluid make it easier to leak? I talked to a couple guys, They said just flush the fluid, as its a design flaw. Thoughts? Im not about to spend 1000$ for the same shitty parts
Old 09-25-2016 | 05:13 PM
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Master cylinder has a seal/square oring that holds the fluid from leaking into the booster. The reason you want to replace both is brake fluid destroys rubber which is what the bladder of the booster is made out of.

Replace both the master cylinder and booster. Fairly easy job, some under dash BS, and bleeding. This particular abs unit bleeds decent manually.
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