Your rear calipers have the emergency brake integrated into their design.
The caliper piston esentially screws in and out.
So, to retract the piston the piston must be rotated while it is being pushed in.
This can be done by turning the piston using a pair of large pliers while pushing it back in. But becareful to not damage the dust boot.
Most parts stores have a special tool which aids in this process.
I would suggest you drop by autozone or your local parts store and ask for the rear caliper retractor lool. They are relativley inexpensive and make this job a breeze.
Here's a helpful link. http://www.toolsource.com/wcar-rear-caliper-piston-retractor-p-90823.html?sourceid=froogle
The tool should look something like this.