rear brakes on kj laser

[lumpy xrt]

hey. i have aproblem with the rear brakes on a kj laser. changing the rear pads, i cant seem to push the piston back into the caliper to accept the new pads. have tryed doing it with a g-clamp and piston compress tool with the bleed nipple in and out and nether does anything.
and tryed with the handbrake cable off.

its not a twist caliper, so it doesnt screw in.

im stumped! i need it done fast, because its my girlfriends car, and it has a brake line clamp on it now so i want it done asap for her saftey.

thanks!