After much troubleshooting: Replacing the brake fluid, disassembling the rear caliper completely and cleaning, pressure washing components, I have run out of ideas on this brake problem. the center piston on the rear brake extends just fine with front left brake rotation while the brake is engaged. the problem arises when the brake is let off, the center piston does not retract. even with the caliper removed and a clamp applied the brake does not return to the housing. *side note* when the brake bleed nipple is loosened the piston retracts into the housing easily. I am not sure how to proceed, any ideas?