If you're talking about the fill valve, I've never seen a fill valve that seats against an oring. Usually the check valve is some kind of hard material that butts up against a smooth, hard valve seat. Think "1377 valve" for example. What keeps the air from leaking out? The contact between the hard check valve and the valve seat. All it takes to develop a bit of a leak is just a bit of crud getting in there and not letting the check valve push up snug against the valve seat. With a good cleaning, removal of any corrosion, etc. it should be good to go. Another possibility, since it sat for a number of years, is that some kind of corrosion may have built up in that area. Pulling it apart and cleaning it is the best bet for getting it up and running again.