2012 EC300R Rear Brake Problem

[Aggie]

I've been having a problem with my rear brakes for about two weeks now and it's frustrating me to no end. Hopefully someone here can help me out. My rear brake piston wont return after being pushed out, causing the rear brakes to drag. I've rebuilt the MC, checked the brake line, and checked the caliper seals.
I've narrowed it down to something in the MC. It's not the brake line, since I can push in the rear piston with just the MC detached. The only thing I can think I may have done wrong is put in one of the MC piston seals backwards (both are cupping in the direction of compression), I figured the one on the bottom is a secondary seal and would cup in the same direction as the first. Any help would be greatly appreciated.