Ditto on the Hydrovac. Your brakes most likely are vacuum assist fluid brakes. If by some fluke you had air brakes, you'd have a compressor on the side of the engine, and prob. a brake release knob in the cab.
I agree that the cannister you found is a vacuum canister, meant to assist the braking and store a little vacuum for a few stops if the engine quits.
Rear brakes use return springs like the front. Don't know the history of your truck, but these springs definately rust and break.
On the other hand, my '44 sat for just two years (inside), and had stuck rears. We pulled the drums (with a lot of effort) and the springs were fine. I think the hubs/brakes just get crudded/moistured up and freeze. Not much to lose by "jarring" them loose somehow - you may avoid having to pull the hubs (lots of work).
Of course, unless you're only going a short distance, it would be prudent to rebuild all the brakes and re-grease the wheel bearings, anyway . . . (what with all the spare time you have, yes? ;-)
-Michael
