Rear disc brakes, ha, not even my dealer knew these were adjusted by working the parking brake. Already have that ancient tool for screwing in the caliper pistons, Cruze had to change this, easy to make such a tool, but needed to keep this thing on the road. So just used a pair of long nose pliers. Two opposing notches on the piston face.
These brakes use the brake lever to activate a ratcheting mechanism to rotate that piston out. Periphery of the piston must be clean and helps to lubricate with brake fluid first so it can be screwed in.
A rubber bushing at the rear is suppose to keep road salt out, but it shortly dries up, this is an age old problem with GM rear disc calipers, that salt permanently binds that screw to the piston, and no way to turn it out, if you do, will never ratchet properly again, only choice is to replace it with a new one.
Using those metal clips on the pad brackets also was a bad idea, a trap for road salt that expands those clips so the pads are bonded hard and will not return to their home position causing over heating and destruction of the rotor.
Has anyone worked on these before, with rear disk calipers, try 40 years, with drums, try 62 years. ABS, also over 30 years, and they lie when they say they do not interfere with normal braking, a transistor shorts out or a valve sticks closed, you get no brakes at all.