Hi.  Just went through this problem and hope my solution is yours.   Symptoms. On my 2000 VFR 800 I replaced all brake pads and bled the system and got a locked up rear wheel for my effort that was traced to the center brake piston not releasing. While locked up, if you open the bleed nipple on center piston, fluid under pressure escapes and the wheel spins. Going back through the brake circuit and pumping the brake to charge the hydraulics and testing each point for pre