How to unseize brake calipers?

[spinners]

Hi guys.I was installing new pads into my starion (rear) and went to compress the piston back with a G clamp and the piston wont budge :(
Does anyone know how to unseize or am i better off just binning them and using another set?
I am running 5 stud brakes etc and have 1 spare off a 4 stud model.
The pads are the same and the caliper looks the same apart from the disc size.
Can I use the 4 stud ones?
Cheers :D

[Cookiemonster]

The rear pistons screw back in.

See this thread:
http://forums.austarion.com/viewtopic.p ... highlight=

[spinners]

Cheers cookiemonster.I thought I was doing something wrong :P