Check for a problem in the master cylinder by opening a bleeder when the brakes are dragging. If that fixes the symptom, the master is the problem.
On the early cars, it was possible to adjust the pedal travel with a stop at the front of the brake box. Sometimes, the master cylinder would stick, and people would adjust the play out of the pedal. If the adjustment was too tight, it prevented the piston to open the master cylinder return port, thus keeping pressure in the system all the time.
The other thing to examine is the balance bar angle and whether it's free. See
here.
If you still have the cast iron (VW) master cylinders, I strongly recommend updating to the later system. It's fairly involved, but worth the effort.