The rear caliper piston has to be turned wile pushing it in. The piston has to be turned clockwise to get the piston back in to the caliper. You can buy a special tool at your local auto parts store or you can try and do this manually.
Retract the brake caliper piston (2) into the brake caliper bore. Use a spanner type wrench to turn the piston (2) clockwise until it bottoms in the brake caliper bore and align the piston.