How hard is it to replace a very old sight glass on the side of a galvanized pressure tank. Over the years it has gotten so stained with rust that I can hardly see through it to check on level in the tank, and seals leak for a week or two in the spring when I start up the system for the season at my summer camp. I'm worried about the glass being stuck to the seals and breaking when I try to remove it for cleaning, and I don't know what kind of seals would be involved. thanks for any advice.