My brakes lock up and I don't know why. I recently was out for a short drive and felt like the brakes were dragging. I stopped and they seemed to free up. when I got back to the garage (a mile or two away) they were locked up again (I could drive - creating heat and smell, but could not push the car). I jacked the car up and found that I could turn the rears, but that both fronts were unable to turn by hand. Lightly pressed the brake pedal and it immediately freed up to the point I could rotate the wheels while up and it rolled it easily. I have a recently rebuilt master (original single circuit) and servo. What else could it be? Any help would be appreciated. Thanks, Josh