Changing out rear break pads

[bjack]

I have an 02 GT, and decided to change out front and rear break pads. I got the front in with no problems, but I could not get the rear caliper piston to compress in order to install the new pads. I tried a "C" clamp to collapse it but it refused to compress.

I noticed it has two notches on the outer edge, does the rear require a special tool? I'm thinking maybe it needs twisting and pushed at the same time........never had this problem before and a little confused on what to do next.