Thought I'd pass on this story. My house is 76 years old and has a steam radiator system with an ancient Bryant boiler. At the very end of last heating season, the boiler started flooding, so I shut it down and the woodstove carried us through the last couple weeks. Last week, I had two plumbers come out to assess it and quote the repair. One guy I've dealt with several times, he's been reliable and honest. The other is the local "expert" on these old steam systems. Both did simple tests (though they didn't break any unions to isolate the issue) and both said that the feeder valve was bad. They recommended replacement of it, the low water level controls and, "while we're at it", all the pipe that connects it. Neither one would quote it firm, but estimated $1200 and $1500. The "expert" also suggested that, rather than dump money into this old thing, I might want to consider a new, more efficient boiler. 5 grand plus for that. I cringed. This can't be. Since I'm relying more and more on wood for heat, I didn't want to dump this kind of dough into the boiler. So I studied the water feed system. Thank god for the internet. Found the manual for the auto feeder valve and read it. Did the tests they suggested. And it sure looked to me like the feeder valve was fine, but the bypass valve might be the culprit. So I pulled it, put a new washer and o-ring in it, reassembled, flushed the boiler, and refilled. Voila. The water came up, the feeder valve clicked, and the water level stopped exactly where it should. And stayed there. Yesterday, despite 94-degree heat, I fired the boiler up to make sure the level held. No problems. Total cost of repair: $2.18 . It certainly pays to overcome the intimidation factor of these big systems and learn how they work!