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Problem bleeding brakes after replacing master cylinder


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2017 GMC Sierra 1500 Denali 6.2L

I replaced my brake pads. After replacing all pads and compressing the calipers, I realized the the master cylinder reservoir was overfilled and by me pressing the calipers back in it cause the seals to leak. The seal in between the master cylinder and the brake booster actually blew into the booster but I was able to get the seal out. I replaced the master cylinder and bled the system with 2 gallons of brake fluid. Brakes are still going to floor, stabikitrak light and Brake light both came on. I’m not sure what to do. 

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2 Answers

Well it also has an ABS system which could have been damaged with that back pressure. But I'll tell you the truth. It's almost impossible to believe the air out of one of those when you change the master cylinder, unless you have a scan toilet that allows you to do ABS bleeding so you're probably best to pay a mechanic to bleed the air first with the scan tool

So I did bench bleed the master cylinder, and I did put an OEM one on. I also replaced my brake booster and flushed the brake lines with a mighty vac. I have a firm petal now, but my breaking power is TERRIBLE. I don’t know what else it could be! Do you?


Did you bench bleed the master cylinder? Aside from that, as Scotty says you're probably going to need a bidirectional scan tool that is capable of bleeding ABS systems.

To avoid this kind of problem in the future crack open the caliper bleeder when compressing the piston to fit new pads. That way fluid will be forced out of the bleeder instead of back through the brake system.