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Advanced Member

United States
250 Posts

Posted - 05/11/2021 :  09:23:33 AM  Show Profile  Visit willism's Homepage  Reply with Quote
Got a 2015 Chevy 4500 what would cause the rear brake lines to swell and start applying the breaks on its own? We changed pads and calipers couldn't get it to bleed came back in in the morning they bleed just fine but noticed the master cyl gasket was all swelled up replaced that drove it for a little bit all seemed fine the driver took it and got about 6 miles before it was starting to apply the rear brakes again towed it back next morning got in it and it drove fine until 5 or 6 and started locking up got under it and saw both lines were bubbling up I have never seen this before any ideas? Thanks

Active Member

47 Posts

Posted - 05/11/2021 :  10:14:58 AM  Show Profile  Visit 671Fan's Homepage  Reply with Quote
That really sounds like chemically contaminated fluid. The master cylinder gasket isn't under pressure so something made it swell up. Hoses are failing also. I would be surprised if every rubber part in the system is not damaged.
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Top Member

867 Posts

Posted - 05/11/2021 :  10:49:36 AM  Show Profile  Reply with Quote
Transmission fluid mixed with brake fluid will cause all the rubber to swell. That's going to be expensive.

US Army retired CMBT
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Senior Member

70 Posts

Posted - 05/12/2021 :  6:03:26 PM  Show Profile  Reply with Quote
Easy check for contaminated fluid is to remove the reservoir cap, if the rubber is swollen the fluid is contaminated. If the fluid is contaminated every single part that contains rubber must be replaced. Master cylinder, hoses, calipers, and the abs hydraulic unit.
I've seen the hydroboost unit fail internally, brakes can self apply while steering.
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Brook Peacock
Senior Member

116 Posts

Posted - 05/13/2021 :  05:03:13 AM  Show Profile  Visit Brook Peacock's Homepage  Reply with Quote
That needs a lot of work.
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