Hello I will help you with your question,
The rear caliper pistons require a tool that puts a little pressure on the piston as you turn it in to allow it to screw in. It is possible for the piston to be out far enough that the caliper will need to be replaced but with the correct tool you should be able to turn 99% in unless the caliper has been damaged
Click here for the procedure
Look this over and let me know how I can help you with this procedure
what is the special tool called. how far do u turn the piston back to put in a new set of brake pads.
The special tool is a brake piston caliper turn in tool and in the procedure I posted above is a part number 206-010 with adapter 206-026.
Here is a link to a site that carries this tool but you should be able to find a less expensive version of the turn in tool at an autoparts shop near you
The important feature you need is the portion of the tool that pushes against the outside of the caliper to put pressure on the piston as you turn the pistion to screw it in.
Glad to help and please let me know if you have further qquestions about this