1997 A8 brakes lockwhile driving
 

While driving 70mph all 4 brakes started locking up/pulled over,all 4 rotors
had minor bluing,no warning lights came on.
Brake pedal like a rock with engine off.Disconnected ABS then reconnected
no change,30 minutes later (engine off) had pedal again but very long travel.
Drove car for 7 days with mushy pedal,now brakes starting to lock up again.
Anyone have an idea what this is the problem?
jeff
973-600-8855

Brake Master Cylinder
 

Popular Mechanics just had an article about the same thing, although it wasn't an Audi. The brake master cylinder was bad. It wasn't returning to the resting position. So, the pressure was not releasing off the pads. The pads were still pressing against the rotors. Eventually, the rotors overheated and locked the wheels. After sitting a while, the master cylinder would eventually return to the resting position, which released the pressure off the pads, and the car was drivable again...for a little while. Solution was replacement of the Master Cylinder. Could this be your problem?

I'd tend to agree with the master cyl thing. If it doesn't release all the pressure, the pads drag and get hot which heat up the fluid and it expands, forcing the pads against the rotors harder, which gets them hotter yet............
One thing to check first would be the booster pushrod depth into the master to be sure it isn't keeping the master cyl pistons from retracting to uncover the ports to the reservoir. Because you mentioned a mushy pedal, it is possible the booster is at fault and not the master itself.

A good way to test the theory is, whent he brakes are "locked", crack open a bleeder screw on the caliper OR crack the lines loose at the master and see if the brakes free up. Also, if the brakes are dragging, try loosening the mounting nuts that hold the master to the booster. If that frees them up, I'd highly suspect it's the booster.