Get the rear wheel off the ground and apply the brake & release . If the rear wheel does not release and it is stuck , loosen the caliper bleed screw and see what happens . Do this a couple of times to see what happens . If it does not release , the problem IS the caliper & go to step 1 If the caliper releases go to step 2 .
#1 . Pull the caliper off the wheel and push each piston out (w/the master cyl. pressure) to see which piston is sticking . If you feel capable remove all pistons and inspect . while you are doing all brake work , use Brake-Kleen only . No petroleum products. When you go back together , go to an old style auto parts store for some brake assy. fluid and reassemble.
#2 Rebuild brake master cyl.
Be deliberate in all steps and tests .