ZEUS+

Have any luck ? To rule out all the lines, you can crack open rear line at the pressure regulator/bias valve. A pain in the a$$ but it will help isolate. If you suspect the ABS system, remove fuse 16 and see what happens. Usually with an abs failure the light will be on. Our systems are three channel, with three valves. Abs valves work in this manner:
In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.
In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder.
In position three, the valve releases some of the pressure from the brake.
The system cannot increase or build pressure higher than what the master cylinder produces.

Dannyfumi

When I open the master cylinder bleed valve the wheels unlock. I pulled the fuse and relays for the ABS and it changed nothing, still locks.

Danny

ZEUS+

The problem is master, rod adjustment or booster as previously suggested. Does master sit flush with booster before tightening ?

Dannyfumi

Zues-
The master does sit flush with booster. I replaced both the booster and master. As far as the rod adjustment I followed the WSM process.

Danny

ZEUS+

Somehow, something in that area is applying the brakes. If you loosen the master from the booster, are they locked ?

ryalex

What was the resolution of this problem?

I'm having the exact same thing on the 1986 928S I bought last week. Both rear calipers start dragging and smoking the pads within 15-20 mins of driving. Rebuilt the master cylinder, bled the system twice, changed out the booster, even tried disconnecting the ABS from the rears. Still smoking and locking up. It does release after a few hours.

Only thing that I don't know if mech tried was readjusting the rod in the master cylinder.

jthwan22

I have the same problem but it is the front locking up. It stop my SITM trip this year.

SMTCapeCod

Did you get to the bottom of the problem?