What is a G clamp? I have never heard of this before.

Yes their is a special tool to wind the rear calipers in and push in at the same time ,if no tool then use a G clamp and apply a bit of pressure then turn piston with a pair of water pump pliers ,better if someone turns the G clamp whilst you turn the piston ,but its easier to buy a cheap tool from the auto factor

You will have to use a c-clamp or brake piston compressing tool to get that piston back in, while you have the brake master cylinder fluid reservoir cap off.

you will want to use a C-clamp for this. Take the c-clamp and an old brake pad, put the pad against the piston and the non-mobile end of the c-clamp on the back of the caliper. Turn the moving portion of the C-clamp in until it pushes the pad into the piston, and pushes the piston completely into the caliper. I hope this helps.

sounds like you will need to replace the caliper.

apply pressure to the c clamp as you undo the hydraulic hose nut(turn undo/turn undo so as not to allow air in system....)

First, take a look at the piston, if it's flat or cup shaped, put an old pad at the piston and use channel locks to squeeze it back in, opening the tool wide to get it to catch the back of the caliper.
If....the piston has what looks like an "X" or cut outs, this is for a tool that spins the piston in with a special tool. (Subaru's and some other cars use this)
After you are all done and pump up the brakes, re-check the brake fluid level.

Use a "C" clamp. Put the threaded part inside the piston, and the other part behind, and screw it till it is flush. Or do you mean you tried that, and it just won't move?

usually with a special tool you can borrow at autozone. because the rear brakes are usually the parking brake. you have to screw them back in. if there is a cross pattern or some type of groove in the piston will confirm this.

A SIX INCH C-CLAMP, PURCHASED AT THE AUTO PARTS STORE, IS NEEDED TO PUSH THE PISTON INTO THE CALIPER AS LONG AS IT IS THE FRONT BRAKES THAT YOU ARE WORKING ON. IF YOU ARE REFERRING TO THE REAR BRAKES, THEN THE PISTON MUST BE TURNED COUNTERCLOCKWISE(TO THE LEFT). HOPE THIS HELPS. PLEASE KEEP ME POSTED. THANKS FOR USING FIX-YA. GOOD LUCK

use a pair of pliers.put each side of the jaws of the pliers in one of the V-grooves of the piston and turn clockwise whle pushing it in