My cousin's got a GMC Jimmy that's having some brake problems that I wanted to get some opinions on. In normal driving, the brakes work fine to a stop. Once stopped however, the pedal slowly loses pressure and goes to the floor.

Sounds to me like maybe a vacuum leak in the booster? Any way this could be the MC?

Thanks guys,
Baker

 

Not a vaccuum leak, but it is a leak. You're either leaking the fluid externally, in which case you'd see the drips and your fluid level would be dropping. The more likely case is, if you're not losing any fluid out of the resevoir, then the master cylinder is leaking internally. The fluid is bypassing the seal, which is why the brakes work fine until you hold your foot on the pedal, as the fluid leaks past the seal the pedal slowly drops to the floor, but as soon as you pump the pedal back up it's fine, until it slowly drops down again. You need a new master cylinder, or you can buy a rebuild kit for the original one if you're up for it.

 

Sounds like that would be logical. I'll have to ask him if he can get pressure back by taking his foot off and then pressing again. Thanks a lot for your help man.

Just for the sake of asking, anyone else have any ideas? Either agreeing or disagreeing from the above is fine...just trying to get a couple opinions before telling him which part to buy and then possibly not having it work.