I am doing a brake job how to retrieve the cylinder back so I can install the new brake pads?

I am doing a brake job on my car. It has disc brakes all the way around. I got the front ones done and now I am doing the rear but for some reason I am unable to retrieve the cylinder back so I can install the new pads. I have done as much loosen the bleeder to release pressure but nothing is happening can you give me a few tips what I am I doing wrong.