The rear calipers twist in, you can usually rent the special tool from autozone or you can do what I did and buy a full brake piston tool set from harbor freight for like 50 bucks. It comes with everything you need to compress the front and rear pistons.
As Fuel said, they screw in. Believe me, get the tool. Makes the job a breeze.
Both of these guys just answered my question and backed my thoughts exactly. Best advice is to rent the tool for free from an auto parts store as no need to buy one unless you do brake jobs often, but even then why not just rent it for free?
Second do inspect the caliper to make sure that it is not damaged from excessive pressure. Also since you opened up the bleeder I am sure you already plan to bleed all the brakes to make sure to get all the air out of the system.
Good luck and keep us posted.
(11.)2012 GT500 silver with red stripe, SVT performance package, and Recaro seats.
2018 Honda Civic Type R
Past Mustangs in order and color of text.
(1.)2000, (2.)2004, (3.)2006GT/Auto traded for (4.)2006GT/Manual, (5.)1996, (6.)2011GT, (7.)2003GT, (8.)2006GT, (9)2013 Boss 302, and (10.) 2002GT
(I have owned a few non Mustang cars over the years as well, but always try to have a Mustang with it or come quickly back to a Mustang!)