S4 Vacuum brake problems?
 

Ok i went and got my fc today. I love it. Anyways the only problem i seem to have found, is that when i hit the brakes they are as hard as a rock. It seems when i downshift and use them though, they get soft, but to only cause vacuum problems (i think). It tries to die out while on the brakes, like a vacuum hose is loose. I checked them the best i could, but its dark out so i have to wait until tomorrow. Has anyone else had this problem? I think possilby a Brake booster, or a master cylinder? The guy has receipts of everything done to it in the past 25 years. A new brake booster was fitted not to long ago. Any help guys?

i'm not 100% sure how similar the FC setup is to the FB, but several years ago, my brother rebuild his car (an '83) and he installed the one-way valve backwards in the vacuum hose for the brakes. his car exhibited the same behavior you describe. he reinstalled the valve the correct way and the brakes haven't missed a beat since. maybe you can look into that.

Ok i just tested the valve and its fine. I even installed it backwards, but that made it worse. The booster is new. Maybe the master cylinder now? Damn..

I'd still say that valve is sticking. Try cleaning it up with some carb cleaner, make sure it moves freely (I think it should rattle around IIRC), and then try again.

Other than that, I think a torn diaphragm in the booster would be the only other probability.

Good luck... :)



.

i've been thinking about his problem for days, but couldn't come up with any other ideas because every other issue i could think of (hydraulic or hardware) "should" leave him with a soft (or normal) pedal effort, not a hard one. wouldn't a torn diaphragm still leave him with a soft pedal?

I don't believe so. I think that if the diaphragm were torn, you would completely lose the vacuum assist, which would leave you with a hard pedal.

gotcha! :) thanks.

It sounds like a defective unit there is a valve inside the vacuum booster also .. If it misses like hell when you push the brakes ,, the diaphram is leaking . At best when the brakes are applied there might be a slight hesitation then level out again ,, If the hoses are good then it is loosing vacuum through the booster ..:)