Are you planning on separating the 2 halves or just removing the pistons?

if you're going to store them that long, you should consider disassembling them, protecting them with a light coat of common non-silicon grease as metal parts only, as the rubber parts will deteriorate by the time you need them. degrease and rebuild when needed.

Link --> Brake Assembly Fluid

Do a search on the name, it's available many places.

It's easy enough to use the right stuff for the job. I bought a similar product years ago and still haven't used it all up:

Tatra Mike

Seattle, Washington

1985 244 "Alfsen" (wife's car - the good one)
1984 245 "Buster" (the kid's car, now sold)
1985 245 "Cosmo" (parts car, sold off for move to WA)
1985 245 "Daisy" (back seat down, full of tools, the work truck)
1985 245 "Earl (CA vehicle 'retirement' program)

Can you tell I prefer the 1985 model 240? Better headlights than
the plastic 1986+ and nicer belt adjustments than the 1984s and earlier!

I've sprayed them with a silicone lube before installing the pistons and dust boots to prevent them from rusting while on the shelf. I don't expect the light coating of silicone to interact with the rubber or future brake fluid.

eg. http://www.canadiantire.ca/en/pdp/motomaster-silicone-lube-300g-0381504p.html#.VHXBnWfiv-0

You may require some pretty high pressure to pop the seized pistons so be careful. I think Haynes gives directions (don't have it with me.)
--
1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

Look around for McKay brand brake assembly lubricant. It's safe for all them rubber parts and has a rust inhibitor.

Car Quest used to carry it. Advance Auto is buying them, so you might find it in their now "combining" stores.

Phil

you should brake assembly grease, just for that. but, it's hard to find. the silicone will react with brake fluid and that's a no-no. good luck, chuck.