Rear Brake Pads HELP, can't compress piston
 

I did the front pads yesterday, pretty straight forward.... no issues compressing the piston using a tool I picked up at Auto Zone.
Doing the rear pads today, and compressing the piston is extremely hard. It's taking a ton of force just to move it a few mm.
Any ideas?
I've take the cap of the brake fluid res. up front, and the E-Brake is OFF.
Any ideas/help would be greatly appreciated!
RT4

I think the rears have to rotate as the the piston is pushed back into the caliper as this resets the adjuster. It requires a special tool.

Last time I did rears on my MCS I bought a special tool although you can use pliers, apparently.
The rear pistons have to be not only compressed but turned as well...Can't remember if they're both the same way, though.
I'm sure there must be posts on here that can tell you exactly what to do.
Edit: Totally agree with Kenny.

Ah ha! The search engine is my friend... you have to TWIST it!
Whew.

Thanks guys.... I used some channel locks and twisted a little, compressed a little, repeat a few times.... good to go!