Venting is a big priority with a steam system. Not only for the basement main pipes but also the radiators as well. A good friend helped me determine the proper venting capacity of each radiator and I put Gorton vents on each one according to its capacity. Mostly sizes D, C and #6. I then tapped three of my largest radiators for an additional air vent slightly below the original as per 'John W. Schulz' mentioned in Dan Holohan's The Lost Art of Steam Heating book (Pg 112). Not only do they help vent those big rads, but now I have the pleasure of seeing a sweet pair of "Double D's" staring me in the face when I round the corner to the living room from the hall. Lucky is the man with a set of D's in his home.
On the Mains are the big Gorton #2s.
The vents have really helped out. I get all my rads to heat across now. It was a struggle to get that sort of solid heat across prior to the new near boiler piping, the venting and of course the serious BTU's put out by the Axeman.