Stuck caliper. Often caused by the rubber hose supplying the caliper swelling inside and acting as a one way valve. This sounds like a good time to do a complete brake rebuild with rotors, loaded calipers, and hoses. I did this in the rear of my '83 shortly after I bought it. The parts ran a bit over $200 and the labor took me all of an hour and a half with just hand tools (in an abandoned parking lot-apartment living sucks sometimes).

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.