On rear brakes with an integrated parking brake, you cannot just push the piston in - you have to wind it back. Here is what caliper piston windback is and why it is needed.
Why some pistons screw in
Rear calipers that house the parking brake use a threaded piston: the parking brake mechanism turns a screw to push the piston out mechanically. To retract it for new pads, you must turn the piston clockwise while pressing - it screws back in; see new pad won't fit.
Why you cannot just press it
Pushing a twist-in piston straight with a C-clamp fights the threads and can damage the parking-brake mechanism; see C-clamp on a caliper. It simply will not go in far enough for thick new pads without turning.
The windback tool
A caliper windback (cube) tool engages the slots or pins on the piston face and turns it clockwise while applying pressure; see caliper compression tool. Some pistons need the slots aligned so the new pad's nub seats.
Do it right
Identify a twist-in rear piston, use the windback tool, crack the bleeder while retracting, and align the piston. Then fit quality pads and hardware.