How to collapse rear brake calliper on a 2005 cadilliac deville. I am using a c clamp with pressure and it wont budge with the bleeder cracked open

Rear disc brakes are different than the front, the piston has to be turned in with a special tool, you should see slots or holes on the surface of the piston, that's where the tool fits in to turn them in.

The rear caliper piston,turns in,with slight pressure on it,one side will turn right handed,to retract,and the other side will turn left handed to retract,the auto parts store sells the tool to do this with,it is a square tool,for sides,with different combinations of pins that will fit the caliper piston for your cadilac.The tool fits on a 3/8 ratchet,I hope this helps you,thank you.