Tony, are you talking about the tank overfill protection valve?
If so, it is now required by law and the old type tanks are no longer legal to refill. Most Propane dealers won't fill them. The old style valves have a round handwheel.The new valve have a more three sided handwheel and are supplied with all new tanks, but they can be backfitted into the old bottles.What you have to look for is the manufacturing date on your old tank. If the tank is steel or aluminum the life certification is 10 or 12 years (it will tell you on the tank). If it is near the end of its life, the certification will run out and the dealers aren't supposed to fill them either, (though some do). When that happens it's not going to matter how good the tank looks, and you will have to replace it.If your tank is fairly new and the refit isn't too expensive go for it. If not, Replace it.I had the old style steel tanks in Mexico and had no problem getting them filled there, but had to change them when I got back up here. I was going to upgrade to aluminum until I got sticker shock so I ended up getting new fiberglass tanks and they are great. They cost less than Aluminum, are much lighter, Have a 15 year life, and are transparent enough that you can see the propane level.