Get the caliper piston pushed back?

This design has the parking brake built into the rear calipers. The parking brake cables pull on the levers on the back of the calipers. Each lever rotates a threaded screw that goes into the backside of the piston. Turning the screw pushes the piston out for the parking brake function.

It's because of that screw that you can't just push the piston back in by hand to reset it. These have to be rotated to wind them down that screw.

As long as that lever is attached, all you have to do is rotate the piston, then the screw will pull it in to reset it. If the lever has been removed, rotating the piston will just draw the screw in, then you can push the piston back in by hand. There's an o-ring around the screw. Expect to see fluid leakage there if that screw gets drawn in too far. Never use a c-clamp to try to press any caliper piston in. Front pistons can be retracted with a large flat-blade screwdriver as a pry bar before the caliper is removed from its mount. If the piston won't move with anything other than a c-clamp, debris has accumulated behind the piston. That will lead to dragging and overheating brakes. That caliper must be replaced or rebuilt.

The second photo shows two different piston resetting tools. The bottom one just rotates the piston. The set in the red case also pushes it back as it rotates it. The two red arrows in the first photo are pointing to two notches in the piston. Mating pins on the resetting tool grab those notches to turn the piston.

Check out this dandy video for more information:

https://youtu.be/j8YwfSKEzZo

This article may be of help to others researching this topic:

https://www.2carpros.com/articles/how-to-replace-rear-brake-pads-and-rotors

Forgot to mention special tools like these may not be a good investment if you're unlikely to use them very often. Instead, the people at many auto parts store will borrow them to you or rent them at very low cost. In my city they make you buy the tool, then you get a full refund when you return it. If you choose to keep a tool, you still return that used one, then they give or order you a brand new one.