Here's the deal : my brake pedal sinks slowly when applied. Now, I know that most of the time this is caused by an internal leak in the master cylinder. I have no fluid leak anywhere else and my fluid level is fine. The only problem here is this situation doesn't happen all of the time. Some times the brakes work fine (sometimes even for weeks at a time) and all the sudden this happens. I can still stop when this happens, it just takes a little extra pressure. It has no rhyme or reason to it happening, it just does it. Could anyone tell me what I could do to check (without dis-assembling the master cylinder). Any help would be greatly appreciated.

 

Does it only happen when you have the AC on by any chance? If so, the AC will draw enough electrical load that the brake booster vacuum is affected. Because the brake booster affects the pedal height, a change in vacuum will directly lower the pedal height, that's why you might feel the loss in brake pressure when the AC is drawing load on the system. Next time test and see if it only happens with the AC on, if it does then it's most likely just the AC load that's affecting the brake pressure. The other thing to check might be uneven pads or rotors, they tend to produce that spongy effect on brake pressure as well.

 

Seals are busted in your MC Replace it.

 

:werd: same thing happened to me. the seals around the MC piston are leaking so fliud is getting past the piston. it wont leak anywhere though so theres no way to tell.just replace the master cylinder and everything will be fine.

 

yeah thats what I figured it was. I just didn't think they would work occasionally with that problem. Thanks for the advice yall.

 

I had this problem with my Wifres integra, it would sink to the floor on somedays and not on others, evetually it got to where it did it everyday, once I replaced the MC it went away