Hello, I've been driving a 1973 MGB usa model LH drive with front disk brakes and the tandem master cylinder for almost 25 years now.The passenger side disk is stuck. That is to say, still applied after the brake pedal has been released. I drove it home but with a lot of vibration. It seems the disk is a bit out of round too. I first had trouble with a soft brake pedal and found an oozing flex line at the same brake. While replacing the hose with the wheel off the ground I noticed the disk was difficult to turn by hand. After replacing the hose and a spin around the block it was even tighter. I removed the caliper, separated the halves and cleaned the pistons up but now it's tighter than ever. What would make the piston halves not retract when the brake pedal is released? The Bentley manual doesn't diagram the overall brake circuit but it seems like the 4 way valve (called a proportioning valve on usa made brake systems) is clogged with goo or some return line may be occluded. Any ideas? Replacing both front calipers, turning the disks and replacing the lines, not to mention a master cylinder would be quite expensive even if I did the work myself. Thanks for your time and happy motoring to those of you in a position to do so.