Hanging rear caliper

[Mike Sylvester]

Mike,

Try cracking the bleeder on that caliper when you try to turn the piston
back.
If it returns OK, then something is impeding the fluid.
The pressure from the MC can over come this restriction but there is not
enough force in the reverse direction.

Your problem could be the MC not letting the fluid back.
I've seen this problem a number of times.

Mike Sylvester