When you change rear pads , you must remove the caliper . (If that requires removing the parking brake bracket, then it must be done.)

Here is a sketch showing the piston exploded. The tool (right) fits into slots on the face of the piston and you should be able to screw the piston back to the "home" position.

In lew of a tool, you can remove the parking brake nut and screw the stud into the piston as you compress the piston from the front (c - clamp or any other resonable force)

Note that the piston will not return home unless you either screw in the piston or screw in the stud that enters from the rear.

How to retract the rear brake caliper to allow new pads to be installed:

Below is a shot of the parking brake stud on the back side of the caliper. The parking brake lever attaches to this. Notice that it has a hex, which matches a hex in the lever. The nut that retains the lever is missing in this photo so you can see the hex.

When you set the parking brake, the lever turns the stud, which pushes the piston out. By reversing the process, you can screw the piston in. There is a trick, when you turn the stud to screw the piston in...it screws into the piston and starts to disappear...as it does, compress the piston and the stud will reappear (until it hits its shoulder) if you need the piston in more, screw the stud in more.

This system also works to adjust the parking brake when you have run out of adjustment on the cable.

I hope this makes sense.