ABS slowly locking up brakes?
 

Just picked up an S5 TurboII, and it's the weirdest thing, the brakes will slowly grab and not let go, basically getting to the point where they're locking the wheels. After sitting for a while (about half our or so, maybe an hour), it seems like they let go (there's no more pressure trapped in the lines I guess).

I'm 99% sure it's the ABS unit, but was wondering what you guys thought. The pedal is hard as crap, we tried the brake booster connected and disconnected, same thing happens.

Pistons stuck/frozen? Just a wild guess.

I agree with the above. ABS isn't going to apply pressure. It's purpose is to eliminate a particular wheel from locking, so pressure is interrupted. Jack up the car and check every wheel to see if they spin fairly freely. You'll probably find one, or more, that needs attention. Lower front pistons have a tendency to bind up because of the condensation that settles in the bottom of the caliper. Rears will also bind up. Have had both happen on mine. Although remote, it could also be an internal break in a rubber brake hose. It will allow pressure/fluid to push the piston out, but will not allow the fluid to flow back, so the pads stay in contact with the rotor.

A bad brake hose can also do that.
The hose is made in layers and the inside layer can separate & act like a restrictor and/or a valve.
It's a cheap fix.

Most ABS problems will turn on the ABS warning light.

Both front pistons were already replaced and rebuilt. Don't know about rears though, but both front and rear are locking up.

I'm guessing it's the ABS because I had a similar issue with my '90 when I had the ABS in it still, the ABS would engage when I was on the brakes over bumps (I would hear it going), and the pedal would get SUPER hard and I couldn't press it in anymore. The pedal is really hard here now too, and I'm wondering if the ABS valves are stuck shut or almost shut, not allowing pressure out of the lines.

Well, your going to need to post when you find the problem. I'd be surprised if it's the ABS. The ABS will activate and you'll find the pedal hard if you strike a bump that will almost "lift" a wheel off the ground. The ABS will sense that the wheel is turning faster then the others and then energize to slow that one down.
By the way, it may just be mis-stated, but the fronts will have 8 pistons total. Just remove the pads, only a few minutes, and see if all the pistons move in easily. The rears are a little more time consuming, but easy. Just see if the pistons turn in easily.
Not certain about this, but when problem is encountered, how about disconnecting the ABS and see what happens, or drive the car with it disconnected and see what happens?

the ABS is disconnected already, the fuse is out and the relays in the ABS unit are out. With all the issues with ABS units on these cars, I'm guessing they're mechanically locked or shut or something.

I think SureShot is right!
The symptoms you describe are exactly those of rubber brake hose delamination. In effect the hose becomes a check valve holding a few psi, that slowly leaks down. It is unlikely that your ABS system is doing the same (they are rigidly designed to "fail-safe" for obvious reasons: see section 11-43 in FSM). You should trouble shoot the hydraulics by loosening fittings/bleed valves on the calipers that are dragging, and report your results here.
Dave

Crap... new brake lines? Might as well get steel-braided, but crap...

There's no chance the crap is stuck IN a caliper is there? Or IN the steel part of the line? Just replace the rubber lines with new ones (or steel braided ones) and I should be ok?

I hate ABS i disconnected mine.

Before you replace all the rubber lines, drive the car until the brakes "lock up" and then pull the offending wheel. Try to push the pistons back in. If they don't move loosen bleeder to see if the pistons move in. If they move in after bleeder is opened, then internal break in line. If they don't, once again check calipers.

All 4 lines being bad at the same time? Or is it only one brake locking up? You didn't specify, you just said "brakes".

The ABS should be "fail-safed" to allow unimpeded flow should a fault be detected, so that would kind of trip me out if a disconnected ABS would do this.

Sounds almost like the booster has an internal seal malfunction, and the air pressures are slowly pulling (or pushing) the master cylinder piston assy into the cylinder bore. But, you said it does it with the booster disconnected, so???

They're all "offending wheels", both front, and I believe both rear all lock up.

And yes it does it with the booster disconnected.

Even if the ABS isnt at fault, you should still junk it.

I know, it's just a pain to re-do it all, bending the lines and all that crap.

Well, my last thought. How about the rod from the pedal being adjusted incorrectly and not allowing the pistons (in the master cylinder) to return fully? After sitting a while, the return fluid bleeds back into the master cylinder and releasing the pads.

might be. I'll have to check that too.. didn't know they could be adjusted. But the thing is, it just started happening recently, nothing was changed.

well, the ABS is out now, now need to get the lines to reconnect everything, I'll let you know if it fixes it.

I really think it was the ABS because it was locking up more than just one brake.

well freaking crap, doing the same thing with the ABS removed.

BUT it's still locking up more than just one brake, so I'm pretty stinkin' sure it's not worn out rubber brake lines.

Any other ideas? Next on the chopping block is the master cylinder. I'm pretty sure it can't be the proportioning valve because that sucker has no moving parts in it does it?

Sorry if you already said this, but have you checked your calipers? Maybe one needs a rebuild or something. I dont know why the master cylinder would make one wheel lock up. It doesnt discriminate between wheels. It just builds pressure in the system and then thats all routed the proportioning valve to your wheels. If it wasnt the ABS, Id say its something at the wheel thats causing the problem.


Im no mechanic though...Thats just what Id look at and how it seems to me.

dont the 2nd gens only have RWAL anyway?

As I stated earlier, when lockup occurs, jack the car up to see which wheel is binding. You can always put your hand on the rotor to see which one is hot, but be careful. If the pads are tight against the rotor and you touch them, they'll severly burn your finger tips. I'd rather drive the car home and jack it up.
I still can't believe it's all 4 brakes locking up. Let us know.

Yeah, I think so.

It's all the wheels locking up, not just one. All the brakes got hot.

Have u checked the pistons on the calipers. They can sieze up. Might wanna replace em.