Steam systems rust, it's normal, because they're open to the atmosphere. You need to get the boiler cleaned every now & again, and accept that vents are meant to be replaced every now & again.
Valves sometimes malfunction because they're dirty (but good luck cleaning them out), also sometimes because the bit of metal that springs them shut, wears out. Boil it in vinegar for a hour, if that doesn't work, replace them.
The problem with that is it is very hard the find the same one, and they are very expensive. The last one cost me 30 bucks.
What are they, adjustable Hoffmans? Vents normally cost about 1/2 that. At least around here they do...
Anyways, whatever you do, don't neglect this - if you system is releasing steam, it's refilling with fresh water more often, which leads to faster rusting & more crap in the boiler, which will clog the other vents worse/faster.
Also, depending on where in the cycle it decides it's low on water & the valve opens, it can really screw up your system's efficiency, too. Not just by cooling the boiler down in mid-cycle - if it starts filling before the condensate returns, by adding too much water. That leads to water in the pipes & rads, which then leads to pinging, blocked steam, and all sorts of irritating havoc.
BTW, greatest source of info on steam heat, and he's out on LI, too:
http://www.heatinghelp.com/