yep...I've seen that before.

I don't know if what I'm doing is wrong, but starting a decade back, whenever I buy a wheel cylinder, I disassemble it, clean and paint the body with engine paint. Then when I re-assemble them before installing them, I grease the aluminum piston with synthetic/rubber safe grease (I use Super-Lube), and I also pack it between the piston and outer rubber seal. I do this to keep moisture out, with my thinking that moisture is what starts and provides the fuel for corrosion.

Since doing all this extra work, I've yet to replace one of those wheel cylinders prepped that way.

edit - And I also use DOT5 in all my old cars that don't have ABS. I realize that there is a current thread debating it, but I've found that to do it w/o problems later on, the brake system has to have all new seals (beaning new or freshly rebuild calipers & wheel cylinders) and the lines be flushed with solvent before essentially doing a first/fresh fill and bleed with DOT5.