The Workings of a Rear Hummer Brake Caliper
Here you are looking at the threaded shaft of the parking brake mechanism with in the Rear brake caliper. This part here as you apply the parking brake turns outwards pushing the piston as you apply the parking brake.  The this is the part the must be turned back when you need to compress the caliper for new brake pads.
Between part A & B has 3 balls between them that move to a shallower location (see below) as the parking brake is applied and does two things, 1, it moves the piston out, and #2  automatically compensate for the pad wear.
Here is just one of the 3 balls.  And if you look at the empty holes where the other balls would be you will see a deeper area where the ball is sitting and the raised are where the ball moves as the parking brake is applied.
Another angle with out the part that connects to the parking brake cable.
The part that the rod screws into is free to slip as the parking brake is applied. That part normally sits within the piston as shown above.
The top of the piston with out the cap on it looks like this.  In the center of the piston is a bolt that is bolted to the inside of the treaded shaft and hold the piston from coming out.  If you are going to remove the piston, do not remove the keeper that is located on the side of the caliper. It has a cap screw that holds the keeper in place.  The keeper mates with the slot on the threaded shaft as seen above.
There is also a spring and some clips not shown here in the above photos.  They got a little cut up as we cut the caliper in half.