Brakes - 2003 mustang rear discs

I have removed the calipar housing on my rear brakes to replace the pads I cannot get the piston to go in so as to put the caliper housing back on the disk. I have bled the system and tried with a c clamp to get the piston i, it will not move.

http://www.sjdiscounttools.com/kd41540.html

The caliper piston has to be rotated back into it’s bore. Do you see any slots or notches on piston face? The tool above engages those slots/notches to rotate the piston back in.

Tester