I had a yamaha rear brake stick so I took it off, exercised the pistons in/out and put it back. After 100 miles the rear disc had lost all of its centre section paint. The black paint was not just bubbled & overheated but burnt to grey ash. !!!
On the Yamaha, once it had cooled, a kick of the caliper freed off the brake and I spent the rest of the ride with my foot well away from the brake pedal. Ive had a car front brake calliper seize in similar ways. The calliper rebuilt fine with genuine parts (pattern parts didnt fit). I also put lots of red grease between the dust seal and hydraulic seal. It's still ok 4 years later - and gets a regular check! BTW NEVER use silicone grease on brakes it causes the pistons to move out and stay out. Red grease allows enough stiction to pull the piston back after brake has been used.
So my bet is corrosion under the piston dust seals locking the pistons. Its often worse when the brake is warm/hot. The callipers both need to be stripped and cleaned and probably have new seals fitted. Had the job been done sooner, the old seals could well have been ok, but once it's got bad enough to jam the seals will be fecked.
Unless the OP knows what he's doing get it fixed professionally.
The bike probably coped better with a jammed brake than any with "normal" tele forks because the brake doesn't affect the steering geometry so doesn't pull the bike sideways when the steering turns. I dont really like tele forks but this is one case where they would have warned of the brakes dragging.
