msce85

I have a 1988 928S4, automatic, 32,600 miles. I take it out about twice a month for about 30 miles. The previous two times it was driven I’ve experienced the same front brake dragging problem:
Incident 1
1. I took the car out for +30 minute drive, brakes work fine.
2. Stopped at grocery store.
3. When I get back to the car and start it, I sense resistance to movement. Normally, when I put the transmission in drive or reverse (with feet off of both the brake pedal and accelerator) the car will start moving slowly on its own like almost any automatic transmission vehicle. But not now - it won’t budge and the brake pedal is stiff. I try pumping the brake pedal to no effect.
4. I start driving (about 30-35 mph) and continue to notice the resistance to movement and the stiff brake pedal. When I take my foot off of the accelerator the car noticeably slows down quicker than usual, even when drifting down a hill.
5. I pull off the road and stop. Both front wheels are too hot to hold my hand on them, but both rear wheels are only slightly warm.
6. I restart the car and now the brakes are no longer dragging; however, the brake pedal is spongy, going almost to the floor. There is no evidence of leaking brake fluid.
7. I take the car out the next day for a short 1-mile test drive and the brakes are perfectly fine again.
Incident 2
About a week later the car was dropped off at the local Porsche dealer after noting the above problem to them. The brake lines were purged. The mechanic drove the car and experienced no problems. All four wheels were removed, the brakes were bled and car drove again with no issues. (Although it appears that the car was not driven for an extended period and then stopped and started again).
1. I picked up the car from the Porsche dealer and then drove it about 20 miles in interstate & city traffic while periodically hitting the brakes relatively hard- the brakes were doing fine with no problem.
2. I stop to get gas and then the above steps 3-7 are experienced again.
Likely cause(s)?

O.B.1

I would look in the direction of the Master Cylinder,mine was doing the same thing ,changed out the M/C and it fixed the problem
Good Luck with this issue

Mrmerlin

I would suggest that you replace the 4 brake lines with stainless flex lines,
install a new master cylinder,
and a brake bias valve thats screwed into the ABS unit.
Then flush and fill with ATE super blue racing fluid.
Also inspect the Booster for fluid contamination.
Inspect the calipers for leaking seals,
and make sure the pads are half thickness or more
Roger has all of these parts

kevinlieb

At least on my old car (not a Porsche) the brake lines can swell on the insides, causing them to restrict and pinch. I'd replace the flex lines in the front and see if that helps.

karl ruiter

On the '88 there are no dust boots on the pistons. So you can get crud build up and pitting on the piston sides. This can cause them to not retract when they should and drag. On my '88 I had to replace about half the pistons, the other half I just removed and cleaned.

MFranke

I would not drive it until you fix the problem. If dragging and getting hot you may suddenly find the pedal goes to the floor and you have no brakes when you need them. Pumping at that point may not help.

If the brake lines at the wheels are original then they should be replaced no question. I'd also replace the MC.

If was a caliper piston problem, it would likely start at a certain wheel or wheels but would unlikely be all wheels at the same time, or unlikely all front or all rear. That would be a MC problem more likely.

Captain_Slow

My 88 auto just started doing the exact same thing today. It definitely resembles the effect of aged and internally swollen brake lines constricting the return of fluid. However, I already have new stainless flex lines all around.

The master cylinder is a likely culprit, but I've had an ABS problem that might be the cause. For about a year the ABS light would flicker, then stay on steady and throw the red warning lights. Pressing the center lighted button would make it go away. It was a very intermittent thing, seemly associated with a loose sensor connection at the left rear wheel (noting that the light always came on when that wheel hit a bump). This behavior faded over time and seemed to be correcting itself. About a month ago I put the car up on stands and it stayed there for about 3 weeks while I diagnosed and repaired a trans leak. Ever since I put the car down and have been driving it, the ABS light has remained off. Somehow, the time on the stands has "fixed" the ABS issue (I'm guessing tension on the ABS harness with wheel hanging down had something to do with it). But...my ABS system has been offline for most of a year, so maybe the return to constant duty has caused the bias valve to fail (?). Just seems a coincidence that the ABS light goes out, then this problem begins....and Mrmerlin lists the brake bias valve on the ABS as a possible cause. Thoughts?

Dads928

This is exactly the description of the symtoms mine had. Master cylinder.

Replaced it, no issues since...

Captain_Slow

Hmmm...I'm just about to update my thread relating to the specifics of what I did to my car today. You may have replaced the MC and corrected the problem when the ABS system was bled correctly. It could be my MC is going, but doing a Google search for "ABS brakes dragging" indicates this is a common problem if brakes have been worked on and the ABS controller/pump hasn't been bled properly. Air bubbles can get lodged in it and so can sediment. Many people think bleeding ABS systems is same as bleeding old school brakes. There are a lot of ABS pumps out there that have never been flushed properly, and what is happening to my car is what they do when they have air or sediment mucking up the valves inside the ABS "pump". The ABS hasn't been working in my car for about a year - so I easily could have had sediment and/or air in it.

Gary Knox

I bought an '88 back about 12 years ago that had dragging brakes on all four corners. I pulled the calipers off the mountings (just removed the two bolts on each), and sprayed a LOT of brake cleaner around each piston. Got a LOT of old caked on brake dust off, re-installed the calipers and exercised the brakes a bit. Again, with the calipers on, I sprayed the cleaner around the pistons as I could best get it there with the snorkel. The pistons freed up, and I drove the car another 30K miles without any dragging.

Captain_Slow

Gary - If what I did today didn't fix it (cycling the ABS pump on a gravel road) I will do what you describe above. If it doesn't help - I'm pulling the ABS relay. If that doesn't make the dragging stop - attention returns to the MC. This car has lived an easy life (very clean)...I doubt it's fouled brake pistons.