Just adding my 2 cents - I found this thread while desperately looking for help on this issue.

On my 91 Troop, the rear pistons have to be screwed in (as has been said above). There is a handy tool (the "cube"), which takes all the fun out of it.

But if you find yourself in the middle of a job, with no way to go get the cube...here's an easy way to get the job done.

I removed the caliper assembly, and lashed it firmly to the axle using some zip-ties. I was careful not to kink or strech the brake hose. Then I took a large pair of needle nose pliers, and turned the pistons about 4 turns clockwise to get them back & nearly flush with the base of the bore. It is difficult to turn with the pliers, but it will turn. Take your time.

It's important to keep the caliper assy from moving, since you'll chew up the piston top with the pliers, and rip the rubber boot otherwise. Once it's immobile, slowly go to work with the pliers.

Also take note - you only get one shot at reattaching the emergency brake cable. If you turn the actuator to attach it to the cable end (with the pin & cotter pin) - and you let go of it...the piston will do it's auto-adjust routine, as it's supposed to. Now when you go to move the actuator again for your 2nd try, it will not have enough "turn" in it to reach the cable. You'll have to remove the caliper and re-seat the piston in the bore AGAIN. Argh. So do it right the first time or spend another 10 minutes fightng with that damn screw piston! Hope this helps others...