you need to turn the piston in.I think there are grooves in the piston, you
can use a pair of pliers or go and buy the tool to do it. if you have to
open the bleeder screw. but the piston isn't supposed to be just pushed in.
if you can't get it to go in with the bleeder screw out, then you may have
to rebuild or replace the caliper, because it has locked up on you.
Have you noticed a decrease in gas milage, noises coming from the rear, is
there more wear on the pads from the caliper in question then from the other
side?

hope that helps
Jeremiah