I have a couple of North American Boilers from 1963! One of the expanion tanks is full of air. I don't think the valve that adds air to the tank is leaking. I think it might be the Airtrol Tank fitting! Can this fitting cause the tank to be full of air? I have releaved the air and it comes back over night. I don't know how many gallons the tank is but it's about 3' by 8' and has a site glass that empty. The other tank is working fine. This is an Ohio school building.