BMW Repair: audi brakes, audi v8, amouts

Question
1990 audi v8 quattro.
brakes are sometimes very hard to depress. seems as if only one set (front/rear)is working. at other times, they work fine

Answer
The most likely cause for the hard brakes is a bad brake booster.  If the brake booster does not have vacuum the brakes will become hard to depress.  the reason the brakeing is sometimes good is likely because the diaphram in the booster has a small hole in it and is leaking small amouts of vacuum. you may have other problems with brake lines or the master cylinder.  you should have your brakes checked out by a qualified audi mechanic, the fact that they apply only in the front sometimes than only in the back sometimes makes not sense whatsoever unless if has to do with the 4 wheel drive.

-cheers