How does the brake fluid look? If it is dirty flush the whole system, it doesn't take much fluid only time. Avoid the urge to use a silicone based fluid (DOT 5). If your car isn't fitted for it you will wreck the whole show. Are you positive you haven't got a disorted caliper? Alloy ones do get this but they often get sticky first. Check balance regulator if fitted. Check control arm inner bushes and radius rods. I have seen cars pull to one side with worn bushes, under braking load you can get more than normal lateral movement which can cause the car to pull. Just some ideas