'80 300D. New master, calipers, brake hoses.
Brakes drag slightly when warm. If I loosen the master/booster nuts they release. I have free play on the booster rod and there is NO adjustment on this car in the pedal or booster area. The booster is working.

This happened a yr or so ago and the only solution, I could find, was to drill about 3mm down into the master cylinder's piston where the booster rod makes contact. It was a temporary fix. The dragging problem has returned. What now???