As you suspected, one of the caliper pistons on the right side was totally stuck. I took the caliper off and broke it down - it was quite a mess.
Actually to pry the jammed piston out I had to soak the back side of the of piston in hot water and leave an ice-cube in the piston. The difference in temperature was enough I could get the stuck piston out. Still, this was the result:
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The pistons have to be replaced. I'm also not so confident that the rotor can be saved, as the grooves are pretty deep. All that being said, I'm looking at replacing all 4x caliper pistons ($100) + fittings (2x rebuild kit $25), cleaning and restoring the calipers (which also should be plated or painted), turning the rotors ($50 if they can be saved, $200 if not). It all ends up pretty close to the cost of a new, upgraded caliper system ($475 at Delorean Go with the rotors). I guess it depends on whether I can save the rotors, but all being said I'm leaning towards upgrading the brakes anyways since the price delta looks like $0 - $200.
I'm getting a set of digital calipers to measure the rotors tomorrow.
I've read elsewhere that the wheel bearings will need to be replaced since they are stock. Can anyone confirm that?
Is there anything else I should be replacing while I'm in here? The brake hoses and master cylinder have already been replaced.