I tried to replace the Drivers side rear brakes today and met resistance with the caliper piston not wanting to retract. I tried to turn it both ways...no luck. I thought that there might be residual pressure in the caliper but was unable to open the bleed valve.

My passenger side rear brakes were actually the problem. The piston developed an O-Ring problem and lost all hydraulic pressure. So, I decided to replace the pads on the drivers side before fixing the larger problem.

What could be the problem with the Drivers Side Piston? Why won't it retract? Ideas on how to fix it?

Thanks Much,
G