cant get rear brake piston to back in!! help

I've had the same problem. What worked for me is to put the old brakes back on bolt everything together and press the brake pedal. This will push the piston into place and then it should "catch." Then start over and it should go in by turning.

something with seizing like coaramprat said. Since you have turned it, it should have unseized and it just needs a slight push to work again.

Not sure if this is how the brake works or not but thats how i fixed it. I also turned the thing for 15 min with no luck.
 
I just opened the cap to the mc and then used a disc brake piston tool from napa. Attach the tool to a 3/8 socket wrench and apply pressure while turning the piston. Mine took like 40-50 revolutions before it finally gave me enough space to put the caliper back on.
 
Finally got one side back in. I think my problem is the last thing I did b4 I started was use the E brake so I could pop the lug nuts loose. I think that was a bad idea cause I think the wheel needs to turn a bit to fully release the parking brake. So putting this side back together and putting it in drive to get a few r's out of the wheel and then I will tear it back down and try again.

BTW. After the other one finally caught It only took about 5 turns to get it fully retracted.
 
35 for a disc brake caliper loaner tool at autozone! i work there, ase certified, you'll have no problems. if you decide to replace them look into semi-loaded calipers, they'd be the same price as replacing new ones and they'll even refund the pads you bought from them to upgrade! THE CORE is just what you pay if you don't bring in the old part, so make sure you bring in the old part and you wont have to pay a core charge! Good-Luck
 
by the way the e-brake uses the driver side rear wheel on mustangs, also make sure to use disc-caliper slide grease to ensure maximum braking performance! ( the slides are the bolts to the calipers, they slide all the way out adn need to be greased).