Rear Brakes on 1996 Mustang, rear brakes, master cylinder

Steve;
How do we compress the piston on the rear brakes?  We have a tool that fits the grooves, turn and push, but no go.  What are we doing wrong?  We also loosened the master cylinder cap a bit.

Answer
Larry start off by mounting the caliper with out the rotor on, that way you don't have to hold it. Second you have to push and turn at the same time sometimes it takes a little work to get the threads to catch( if the piston was fully extended. Just to say it, make sure you don't have the e-brake on. You should also loosen the bleeder screw, not the master cap. If none of this seems to work, you might need a new caliper.
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