Dragging Brake, No Solution In Sight

1999 Audi A4, 2.8 liter V6, AWD, manual 5 speed, 128,000 miles.

This problem presented itself literally overnight. Starting off cold and driving in stop/start traffic, over the course of about 3 miles the front passenger side caliper will gradually lock up to the point of causing forward motion to become impossible.

If you let the car cool completely the cycle starts again, with completely free coasting until things start to bind up again.

Initially, the passenger front and driver’s rear brakes locked up at the same time. Since these are on the same channel, i took the tires off both offending brakes and inspected. At the rear I noted a rusted tight caliper mounting bolt, upon which the caliper slides. After extricating that bolt, sanding it down with fine sandpaper and reinstalling it with ample grease. I gave it a test drive only to discover the problem was gone at the rear but still happened at the front.

I then took off the front caliper, inspected the bolts, and re-greased, checking for any kind of other interference or drag and finding none, reassembled and took it out for another test only to find the problem was still there.

Aside from starting to throw parts at it, I’m at the end of my wits. Does the assembled wisdom here have any ideas?

Sorry forgot to add. As an additional test, I took the car out for a drive from cold and DID NOT use the hydraulic brake system to stop, only the emergency hand brake. after a 2 mile drive, the rear brakes were predictably hot and the fronts were icy cold, just as it should have been.

You say the weather is icy cold? That could be the problem ---- ice in the brake lines. It could, also, be the interior of the brake hoses have deteriorated and debris is blocking normal pressure bleed-back. Remember, there is thousands of psi pressure is pushing brake fluid toward the brake caliper; but, practically zero pressure to return the fluid toward the reservoir.
Disconnect the caliper end of the brake hoses. Use the brake pedal and pump the brake fluid out. Keep refilling the brake reservoir as you go. If this doesn’t fix the problem, change the brake hoses.

No, not icy cold out, it’s spring here in Detroit and the temps are bouncing between 25 and 60. Even in the darkest depths of January when it was 20 below this problem didn’t occur. I was referring to the car being completely cold, no heat in the engine or brakes.

Check the brake hoses. If a hose is crimped it can maintain pressure on the brake caliper even after you release the brake pedal.

You greased the caliper slides, but you didn’t check the caliper piston(s). How do you know the pistons aren’t sticking in the calipers?

Yes, I’m beginning to suspect the piston is hanging up. When i inspected it there was significant scale around the seal, though no leakage.

After your suggestion, I did inspect the lines and found no kinks of any kind.