May be a stuck caliper piston,which you can check by unbolting the caliper
and pushing the pistons back in their bores.Or,something I've seen on rare
occasions,an internally collapsed brake hose,check this by opening the
bleeder(with wheel off the car)then see if brake stops dragging.The only other
thing I can think of is the master cylinder pushrod out of proper freeplay
adjustment.Hope this is of help.

  Karl