I have had an occasional problem with the brakes on my '89 S4 car dragging. It's been slight and was at first hard to diagnose. There wasn't any strong pull to one side or another so maybe both fronts are occasionally dragging. It usually happens when they get hot. On a 95 degree day they may start to drag after 20 minutes. When I pull back into the garage, they are very hot. When this happened several months ago, I bled the brakes and it seemed to take care of it for awhile. I bled the brakes last week and no cigar; they began dragging the next time out. I have been using Super Blue brake fluid.

Has anyone had these symptoms, and if so what did they do to correct it?

Should the calipers be rebuilt? How hard is it and are kits available?

What about the master cylinder? rebuild? It does not leak externally.

Any help would be greatly appreciated, I would like to remedy next week at the latest.

Thanks
--
Gary Tilson
'89 S4 Guards Red

=============

i don't know if this will help your situation, but one time i had just had my brakes done (pads, rotors and flush) and my front calipers got real hot on the way home and locked up the wheels. wouldn't move. so i called the shop and the mechanic said to crack the bleeder open, just enough to relieve the pressure, but not enough to suck any air back in. so when i cracked them it squirt fluid and i shut them just as the stream was stopping. don't quite understand what the problem was, but they never dragged again.

something to try?

dave

========

Cool, an actual question.

Let's work from the back of your list to the front.

Master cylinder. Usually responsible for not applying the brakes when it fails so not a likely candidate for dragging them. However, one symptom is that the vacuum seals fail at the back and allow fluid to be sucked into the vacuum booster eventually dragging the brakes by hydrolock. A candidate if you are pouring in fluid and don't know where it is going. If it gets bad you will see puffs of some unknown smoke coming out of your tail pipe and that is the brake fluid getting sucked through the vacuum system.

Calipers. They usually don't go all at once and/or aren't variable or moving around the car with random symptoms. Usually associated with a pull when one or more go out and the brake at that corner isn't working so the brakes would feel less effective and the steering wheel would respond so not likely either. Also, the 4 piston calipers are not easy to rebuild so you don't want it to be that. Not really hard just involved.

The heat related part of the syndrome is telling a bit but may just be normal. Brake fluid expands when it gets hot if it has no where to go it will drag the brakes.

The other thing that can be a factor is old rubber lines. They can collapse after time or the rubber liner layer can come loose and occlude the inside. But again that is normally one caliper not both fronts. But bleeding might dislodge something so it could be.

My guess, if I have to guess is two fold. If you have been pouring in fluid and it isn't coming out on the ground or leaking anywhere you can see then you have a vacuum booster full of fluid. You can suck out the fluid through the vacuum connection and see if it makes a change. If you haven't been losing fluid I hate to even say it but your booster may have failed in the same way mine did and it is done. Needs replacement.

Hope that helped.

Jay K.

==============

> I have had an occasional problem with the brakes on my '89 S4 car
>dragging...

One thing that hasn't been mentioned yet: you haven't changed pads lately have you? Some replacement pads don't really fit that well, and need a little work with a grinder to keep them from hanging up and dragging intermittently.

-John White-
'85 928 S

==============

Gary,

I saved this post from Jay Kempf regarding this situation. I haven't fixed mine yet since it's only an intermittent problem, but my car is doing the same thing as yours - brakes start to drag mainly during the hottest weather after 15 or 20 minutes on the interstate. Let us know if you find that this is the solution for your car.

Ed Croasdale
'85 5-spd

Subject: Booster dissection report (longish and probably boring to most...)
From: "Jay Kempf" <jkempf@tds.net>
Date: Mon, 10 Jun 2002 23:02:04 -0400
X-Message-Number: 66

Well, I took the thing apart today.

To recap: booster suspect failure. Symptoms were the brakes being applied by the vacuum alone without input from leg. Intermittent problem that seemed to go away with repeated hard application of the brakes or a cycle of engine
shutdowns to release vacuum. Prolonged highway seemed to make it worse.
Hard on and off brakes and throttle (for 300miles of BRP) seemed to cure it. I had dialed a ton of free play into the pedal and when the problem occurred it would consume all the travel (about 1") and make a rock hard brake pedal
with touchy brakes. The fronts seemed to drag mostly and it would change from side to side. Go figure. Was making me crazy. I also had suspected a blocked front brake line and collapsed brake lines. All have been recently gone through leaving only the vacuum as the culprit.

So I put a new booster in and cleared one final brake line and it's fixed.
This after recent new brake hoses all around and caliper rebuilds. It was due anyway for those.

But tonight I dissected the old one just to find out how it could do what it was doing. The only thing I could come up with is the the pushrod was in two parts and that pressure was somehow leaking and lengthening the overall pushrod pushing on the master cylinder and also pushing the brake pedal and inch back to the stop. And that is exactly what I found. There is a big rubber diaphragm sandwiched between the halves of the casing.
There is a big backup disc to stabilize the diaphragm so that it is sealed at the outer edge but puts all it's force at the inner piston. There is an inner plastic housing that the pedal side and the master cylinder halves of the central pushrod mount into. This central housing has porting at two different levels that either dumps vacuum overboard or directs it from the chamber on the master cylinder side to the piston. The piston is a spool valve arrangement that selects the vacuum source or the dump depending on input on the pedal side. The brains are attached to the pedal side but the
valve body is isolated so that pressure balances it between your leg pressure and vacuum. The way it works is thus. You push on the pedal with your leg. This pushes on the pushrod which pushes through the valve body against light spring pressure opening a valve that starts to pull the front half of the pushrod With the diapragm assembly toward the master cylinder.
There is a big spring in the chamber pushing the diapragm back toward you and there is a little spring pushing on the center of the valve. So the valve itself is somewhat isolated. As the vacuum pressure grows and pulls it moves the little spool valve inside the body closing the port off that is letting the vacuum pressure build. For that pressure (your leg) it moves until it balances it out with the equivalent amount of vacuum pressure and movement of the master cylinder piston. Because that gigantic 10" diameter vacuum is much, much larger than the little master cylinder it amplifies
your leg. So if you are pushing with 20 lbs at the pedal it is doing something like hundreds. If you push more it opens the port more and more vacuum pressure pulls more until the valve moves and it reaches equilibrium again. When you release the decreasing pressure yanks the spool valve back opening the dump port and bleeding the vacuum pressure down til it reaches equilibrium again. Even when it is completely relaxed the vacuum is on full it just isn't pushing on the pedal because there is no input pressure to activate the central valve it is not because there is a rear port. Real complicated way to amplify a signal. I think the pressure sources for ABS systems are much less complicated by comparison. I am not going to believe that they are less reliable anymore.

Anyhow mine failed because one of the rubber seats on the central valve was half off the piston. So sometimes it were working and sometimes it wasn't.
I had put a tone of what I thought was free play in the pedal not knowing that free play wasn't a factor. When I was hard on the brakes repeatedly it probably was just luck the every once in a while the seal would get sucked into position by the vacuum and work till I shut the beast down and the pressure pulled it back out of the bore of the valve. So it was letting vacuum by enough to hold the pedal down and effectively push the valve back toward the pedal until it ran out of travel and then just held down on the brakes. Amazing that it didn't just lock them up. Must have been a very small leak. Just enough to hold the pads against the rotor and start to heat and warp them.

One other thing I noticed. There is no reason to pull the pushrod off the pedal to adjust it. Turning the threaded side of the booster rod is intended for that. I always pull the pivot pin and spun the clevis end of the pushrod and that works but you don't have to. Also this pushrod length has nothing... nothing to do with brake free play. Really. It is a pedal height adjustment. Cool.

So Mr. Trackmeister there is no explanation for why yours is working. You got a problem with your booster and it isn't functioning properly. What you though you were adjusting isn't what you thought you were adjusting and the feel that you don't like is actually something wrong with the internal valving. Just happens that it isn't acting the same as mine was. Maybe it has a limit where it seals back up in the bore so you have some useful travel and as long as you stay behind it it is working but it ain't right. May last a week. May last for ever. May stay the same. May slowly decay into another symptom...

Interesting stuff. Hope this won't start a run on boosters for old buggers...

Jay Kempf

===========

Thanks for the responses when I posted my problem a couple of weeks ago. I have since fixed the problem with the brakes dragging. Conventional list wisdom pointed to the brake booster.

The first thing I tried was a booster check valve since it was relatively inexpensive. No dice. After driving the car until it got hot enough to drag the brakes, I unplugged the check valve from the booster. I drove without boost (carefully) and the brakes still dragged. It appeared that the master cylinder was therefore the problem. This is what Joel Roeder of Northpoint Foreign Auto had predicted all along. We put a new master cylinder in and guess what - no dragging or pressure build up.
--
Gary Tilson
'89 S4 Guards Red