Thanks for the notes, that helped. Unfortunately, I still can't figure it out.
Re water level in the site;
The furnace has a sticker on it marking the proper level, and it is a little less than 1/4" down from the top. Also, as it is, with the water up to a little higher than that mark, with temps in the 20's at night, I have to refill the water at least once a day. If I left it at 1/3, I woul dhave to refill it every couple hours to keep the furnace from shutting off.
Re pressure guage; it's fine.
The valve on that particular radiator is full open, as is the vent.
Even as such, it bangs a lot, but gives very little heat. It is the worst (coldest) room in the house, and I have to have an electric heater in there to keep it comfortable for the 8 month old baby that sleeps there (the knocking wakes him up a lot). I wonder if that loud knocking and lack of sufficient heat are related, but the heating contractor did not seem to think so.
On another subject, in my bedroom my wife insists on keeping the valve halfway open because she thinks it gets too hot with it full open. An ongoing argument for me as I keep telling her to open it full and close the vent (which she also insists makes it too hot). Recently, I have started to wonder if our valve not being full open is affecting the knocker.
Lastly, regarding the level, the rads appear to be level, but I have not used a level to check. Even so, that rad is a kind that is closed on the front and has vents at the top, and is built into the space in the wall. I can't even see how it would be adjusted, but I am not sure.
The contractors that installed the steam furnace are a good company, but it seems that when it comes to steam heat, most anyone I talk to is never really firm on what a rad or balancing problem is caused by & they don't usually have a specific fix. This is not just specific to the people that installed this furnace, either. The previous oil company I used was the same way.
I don't know what email you are referring to since I did not correspond with you. But, thanks for the tip about what that water I keep filling my site glass with is for.