Watch this video from This Old House to learn how to maintain a steam radiator.
1 Close the valve at base of radiator.
2 Turn off the room's thermostat.
3 Carefully unscrew the old air vent from side of radiator; if valve is hot, use thick cloth to protect your hand.
4 Wrap Teflon tape around the threads of the new steam valve.
5 Thread new steam valve into side of radiator; be sure valve points straight up.
6 Remove handle from valve at base of radiator.
7 Use adjustable wrench to unscrew packing nut from valve stem.
8 Wrap graphite packing gasket around threaded valve stem.
9 Replace nut and tighten with adjustable wrench.
10 Open valve at base of radiator.
11 Turn on the room's thermostat.