Well, I released the rear bleeders, fluid came out and I was able to retract the pistons. I tightened them back up, pumped the rear brakes pedal and they locked back up. Repeated the process, but only applied the front brakes, rears stay retracted.
The bike is an 07 with around 16,000 on it. I have change the brake fluid every year, last change in the fall.
OK, so if the bike remained stationary during your test, part of the problem must be between the rear master cylinder and the caliper because unless the bike is moving, the SMC would not come into play, even when pumping the front lever. The middle located bleeder relieves fluid from the SMC, while the top bleeder relieves pressure from the brake pedal. Since the pistons moved when you released the fluid, it sounds like the return bleed hole in the rear master cylinder might be clogged. This exact thing happened to my Interceptor and I ended up rebuilding the rear master cylinder to fix it.
A further test to exonerate the SMC would be to check after pumping up the brakes again and going for a short ride (make sure to use the front brake) now start by just opening the top bleeder. If that releases the rear brake, I'd say your SMC is probably OK. If it's still stuck or dragging too much, then loosen the center bleeder. If that releases the rear brake then your SMC may also need attention.
Also be sure to inspect the rear brake lines for a crimps, pinches or any damage. As pointed out above, the high pressures generated by the lever and the pedal can force fluid past an obstruction that the small release pressure from the flexible piston seals cannot overcome.
