Bad Master Cylinder?

nfisherman · August 19, 2017

I have a 2015 Sierra A/T w/60K miles. It has new brake rotors and pads all around. If I press the brake pedal for a long time, like when backing down a boat ramp, the pedal bottoms out and I lose braking power. I'm not losing brake fluid anywhere. I'm I right to think maybe the problem is the master cylinder? That part shouldn't fail so soon, but it's out of warranty, of course, and something is definitely wrong. I'd appreciate suggestions about things to check. Thanks

DeePa · August 19, 2017

I've heard others complain about extended braking then losing pedal.

Something about the vac pump???

There's another thread about it. Take a peek.

Justin101714 · August 20, 2017

You won't fix this. Your pressing the pedal faster than the vacuum pump can keep up. If you wanna check, take the hose off of the booster that leads down to the pump. Make sure there isn't engine oil in the hose. If there is, replace the vacuum pump and hose, as well as the booster assembly. If there isn't, it's normal and nothing you replace will fix it.

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nfisherman · August 20, 2017

Thanks for the replies. I checked the other thread, too. That definitely sounds like my problem. I'm glad I asked the question. I could have replaced the master cylinder but it wouldn't have done any good.

I saw that a few people said this can't be fixed. I don't understand that. My brakes worked fine when the truck was new. Now they don't. It seems that something is malfunctioning and fixing that should restore the brakes to the way they were. I'm going to focus on the vacuum line and the check valve. That seems like a good place to start because a lot of people have said that it's a vacuum problem and those particular parts are pretty cheap.

nfisherman · August 20, 2017

I found this service bulletin.

https://www-odi.nhtsa.dot.gov/acms/cs/jaxrs/download/doc/UCM507569/SB-10057667-3895.pdf

FL335i · August 21, 2017

70k on my '14. Since I've been commuting a lot in city traffic I noticed that in severe traffic my brake pedal will go almost to the floor. I did read the PIT/TSB for the vacuum pump, but what I cannot understand is the PIT/TSB is for a pedal that gets HARD, not falls to the floor as if it's a bad master cylinder. When the vacuum pump is weak or cannot keep up, the pedal should become HARD, not fall to the floor. I'm with the OP..... something is up here.