What I do is put the caliper in the vice and get a 19in spanner and Lock a vice grips on to it and turn the piston into to the caliper. I use a hammer Lightly to start and work with it with the spanner. Have a go it will work.
where the handbrake cable goes on to the caliper PULL that open like you have the handbrake on FULL and turn the piston into the caliper NOT push it in different to the front calipers.