The Centric brand kit from Rockauto for the 84-87 rear caliper comes with the seal for the inner piston. I've only ever had an 88 piston apart but I would think that the 86 would be similar. It's not too difficult, you just need to be careful not to damage any of the inner workings of the piston when taking it apart. Replacing the inner seal on the piston will eliminate the leak you have.
Here is the thread showing the 88 piston disassembly/assembly procedure, I'd guess pre-88 would be similar.
//www.fiero.nl/forum/Forum2/HTML/137765.html[This message has been edited by seajai (edited 09-26-2016).]