Did you see the valve photos in my FC470 project thread? That should give you an idea of what to look for. I'd say it's very easy to take the valve body apart and check it and re-grease it. I used some standard marine grease I picked up at the local West Marine. I think it was Quicksilver 2-4-C grease. The FC470 manual recommended "Shell Retinax A" grease for the valves, which isn't available anymore but some google research suggests that it was just a standard grease suitable for marine applications, nothing fancy.
You shouldn't have any leaks from the brass intercommunication valves, I don't think they could wear out. Just make sure they are totally clean and then apply a light coat of grease before putting them together again. To clarify, I'm talking about pulling the outer valve cone, not the part that's glued/heat welded to the boat. There aren't any seals or anything in that part anyway.
If you're getting leaking from the first (inner) valve with the flapper diaphram, that's normal, a couple of mine do that too. I don't think it's designed to be an airtight seal, just is there to help you not have the air leak out immediately when you're inflating the boat. It's the outer screw-on valve cap that is the true airtight seal. The positioning of the intercommunication valve will affect whether the air inside the boat gets to the outer valve cap at all, since you can open the valve up to either the left or right side chambers, or both at once depending on the position.
Do you have leaking when the valve caps are on? That's the only thing that really matters.