'73 MGB front brake not releasing,..

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.
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I'm not familiar with USA models of this car, but a quick check might be to open, briefly, the bleed nipple on the affected calliper. If this frees off the brake, then the fault definitely isn't in the caliper. If it doesn't, try briefly opening a bleed nipple on another brake on the same circuit, or a pipe connector. Once you've narrowed down which bit of the system is retaining pressure, diagnosis may be easier.
Hang on: I've just noticed you said that it was even difficult to turn while replacing the hose - so when there couldn't have been any hydraulic pressure. Did you try turning the wheel with the caliper removed? Could it be a very stiff wheel bearing?
--
Kevin Poole
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on the busses wrote:

How about getting the calipers overhauled so the pistons work - they do rust up inside,
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Sorted: new calipers. tried to fix the old ones by cleaning them up but no use. thanks to all.

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