04 GP GT1 Rear Brakes - Compress Piston Q

I started in on replacing the rear brake pads on my 04 GT1 and am unable to compress the caliper piston. The car's driver side rear was on a jack stand, e-brake off, master cylinder cap loose, and chock blocks in place on the front wheels.

The caliper came off easy with the 2 bolts removed. I tried to compress the piston. I removed the rear spring thinking it was putting pressure on the piston some how. I tried to rotate the piston itself but it only went 180 degrees clockwise. No luck compressing the piston. :dunno:

Can some one please enlighten me with a procedure on how to compress the piston so that it will fit over the new pads?

The reason you are having problems is due to the ABS holding pressure on the system. You may need to take the car to someone with an ABS capable scan tool since some of the new model abs systems require that the system be disarmed to bleed the brakes. Also by compressing old fluid back into the system you can damage the ABS by putting dirt into the valving and pistons. Unfortunately, you may need a dealer or mechanic to do the change for you.