Why won't my Brake plunger on the caliper not compress?

there are two ways caliper pistons are made, one you can just use a c-clamp to push the piston back into the caliper; the other piston type must be screwed into the caliper with a special tool (inexpensive and readily available at your local auto parts store), but i must warn you, this "screw type" is very annoying to deal with, sometimes you have to start screwing it in with a pair of channel lock pliers b/c the tool wont fit in the caliper until the piston is most of the way back into the caliper, but be careful not to scar the outside of the piston and not to break or tear the seal