Leaky baseboard and banging

The Harts

I have no idea about steam heat. Please bear with me!!! I have a leaky baseboard. I removed the end cap where the air vent is, and noticed that the leaking seems to be coming from a maid-o-mist. Is this a air vent? It looks Like a two piece fitting one part threads into the end of the pipe, and the other cap is cripmed on with what looks like a round vent on it. Sould the round vent be facing up? This is where its leaking from. Also, the thing hisses constantly. We also have major hammering when the heat first starts, but it is only in the one line to the furtherst radiator from the boiler. All the others are pretty much quiet except for what I think is hissing air vents. How do I fix this?!?! Can I make repairs with the system running or should I wait or shut the boiler down? This is our heat and hot water provider. We also have to refill the boiler every other day since winter has started (granted the heat has been on non-stop). Will I induce air into the system if I removed the Maid-o-mist? Will this cause even more banging?