HELP WITH REAR BRAKES!?

I have a 2008 Saturn Astra XE and I am changing the back disc brakes. Here is my problem... I can not get the calipers piston to compress back! I have the kind where you need a special turning tool. I bought an adapter socket for my wrench at an Autoparts store. I have been trying to turn it clockwise to get to compress and it just wont budge! I have tried to put pressure while turning it and that doesn't help either. Please let me know if you know anything! Should I bleed the brakes a bit? Help!