Johnny,
I am no expert in this area but my guess is that the old brake cylinders were ruined by internal rust (pitting), not external rust. If that is the case, the best way to extend the life of a brake cylinder is to flush/bleed the brake lines often (if you use regular fluid, which absorbs water) or use silicone fluid (that does not absorb water).
Do not consider using silicone fluid unless all your rubber parts are new (in the master cylinder and in the wheel cylinders).
I painted my master cylinder "cast gray" (it can be seen) but did not paint my wheel cylinders. On my suburban, I installed all new brake system parts, so I am now using silicone brake fluid in that system.
Good luck,
Tim
