My rear disc caliper plunger came all the way out( a good inch and a half) when the pads went missing(don't ask). I took it off and even put it in a vice and it will not compress. Do they lock when they come all the way out?
Of course there is no brake lines or bleeder valve attached?
This is an 87 9 bolt if it matters.

 

to get the piston back in the bore u need to screw it in, there is actully a tool for it but i dont think youll really need it..it has to do with the way the parking brake is set up for those calipers,

 

I'd imagine, too, that you could do it as a "rebuild" would. Remove the nut holding the parking brake lever/counterweight on. Remove the rubber washers (lube 'em with silicone grease before reinstalling). Let the adjustment screw come out of the hole where the piston was. Drop the adjustment screw into the piston; the threads are "steep", so it'll screw in quite easily. Then, with the adjustment screw seated all the way into the piston, lube the outside of the piston with brake fluid. Hold the piston "screw side up" as you put it back into the caliper. Compress the piston inside with a c-clamp (but don't block the hole where the adjustment screw will appear!). Make sure the adjustment screw finds its way thru the hole in the back of the caliper. Then, put a block of wood in the caliper to keep the piston compressed, and reassemble the caliper. I forget what the adjustment screw-to-parking-brake-lever-nut's torque value is. The 82-92 Haynes manual would have the torque spec.