2005 Volvo S40 S40 Rear Brake Problem

Brakes problem
2005 Volvo S40 4 cyl Two Wheel Drive Automatic

I needed new rear brakes so I decided to to do them myself since I've done my wife's Dodge Durango several times with no problems. I'm having a problem removing the caliper. I got the mounting bolts off but the caliper won't release the pads from the rotor. I finally was able to push back the caliper piston enough with a screwdriver and hammer in order to remove the caliper but I don't want to do this on the other wheel.

What is the trick to remove the caliper? Am I missing something? Why won't it release the pads from the rotor.

Also, I've found that the caliper piston will not compress with a C-clamp? Is there another tool I can use and do you think I damaged the piston when prying it away from the pad with the screwdriver?