2005 Volvo S40 Brake caliper puck

If the parking brake is built into the caliper, the piston must be screwed in with a special tool. You should never HAVE to use a c-clamp.

If the parking brake is a separate shoe assembly, open the bleeder screw on the caliper, then pry the piston in with a flat blade screwdriver. If it doesn't move, the piston has a ring of rust pits or dirt that is getting caught on the seal. The caliper must be replaced or rebuilt.

Caradiodoc

The rear pistons need to be screwd back in using a special tool for it. Most auto parts stores have special tools to rent. Call them up and see if they have one for your vehicle. If not you would need to buy one. One trick you can try but may be pretty hard to do is get some long needle nose pliers. Insert it on the 2 holes and while pushing in, turn clock wise. It is going to be hard and that is why I now have the special tool ;)