Is it normal for the level in the sight glass to drop more after the first boiler cycle after a long shut down period , than after a similar heating cycle during the heating season when the boiler comes on regularly?
Could this be due to water in the radiators (mud legs I think they are called) just having evaporated and so holding back condensate till it reaches the outlets, or could it also or additionally be low or insufficiently sloped parts of the piping? I wonder about this because if I check the water level in the sight glass a day after shutting down for the season, and then again a week later, the water level has gotten higher.
If it only happens on the first cycle, I'd suspect clogged wet returns. If it continues to happen, my guess is wet steam, bad piping, trapped mains, etc. if the boiler sits for a long time, and continues to fill, you may have a leaking fill valve.