Brake help! - odd problem (as usual for me)

Good to hear of your persistence-based success. The brake booster issue is just another example of dealing with vehicles that are approaching antique status. Parts are just getting to the point of simply wearing out.

I'm getting ready to rebuild my brake calipers so please stay tuned since I may have a few questions myself. On a couple of occasions since the purchase of my 90 S4, the front passenger caliper has not wanted to release after braking. It's been suggested that the master cylinder is the cause, but I'm not sure if that quite adds up when it's only one caliper the problem is happening to.

One of the previous owners had the calipers repainted, so I've been wondering if something didn't get into or around the piston bore in the process that's been causing the issue. Either that, or they were simply painted and not also rebuilt during the process and it's just that time for this part on this old a vehicle. I'll find out soon enough I guess.

Congrats again on getting the issue solved.

I've had a new development regarding my braking issue. Since purchasing the car, the only brake problem I'd had was with the front passenger caliper sticking, but the other day the front driver side joined the party.

I rebuilt the two front calipers recently (new seals etc), but the scenario sounds all too familiar with something that was said in the first post in this thread. When this happened on my car the other day, the outside temperature was also warmer than usual (mid 80's), which was about the same condition as when the front passenger caliper sticking had happened the very first time.

I think I'll try the same and disconnect the vacuum line from the brake booster to see what happens during a warm day drive, but the symptoms are sounding all too familiar so it seems a new brake booster may be in my future too.

You never know, sorry to hear that. We tried connecting and disconnecting the booster and turning the wheels by hand without the booster,,,with the booster, without the booster etc to all 4 wheels. With it disconnected all 4 worked like a charm (although no boost of course.) That started leading us back to the booster. Good Luck!

mm928 wrote: You never know, sorry to hear that. We tried connecting and disconnecting the booster and turning the wheels by hand without the booster,,,with the booster, without the booster etc to all 4 wheels. With it disconnected all 4 worked like a charm (although no boost of course.) That started leading us back to the booster. Good Luck!

Was hoping you would clarify what was meant when you said the following about the brake booster test being done incorrectly the first time. Did you just pull and cap both vacuum lines going to booster (main vacuum line plus smaller vacuum line with check valve)?

- Repeated an earlier test, which was done incorrectly the first time,,,,,- pulled vacuum line out of the booster, capped it off ,,, went for a drive, no drag whatsoever! Release is normal. Just no boost as expected.

Thanks for the well wishes. It'll just be nice to get to a point where I can plan a day trip with the car without having a high probability the issue is going to rear its ugly head again.

Still no joy on this one. Took the car to work today with both vacuum lines to the brake booster unhooked and plugged and could tell that the drivers front caliper had still hung a little on the way home because of that hot brake smell when I pulled into the driveway.

Is it possible that the brake booster linkage to the master cylinder is not adjusted correctly? I've read in the manual that the measurement from the bearing surface to the eyelet center of the connecting rod should be a very specific measurement (within a few mm).

Maybe just a master cylinder replacement?

Just curious how the fact that it mainly happens during warmer temps applies.

The current diagnosis is indicating the master cylinder. I have one ordered and on the way so I'll find out soon enough.