I bought a 1970 VW bus that had this problem. The guy who sold it to me demonstrated his technique for checking the tank. He would stand in the middle of the cargo area and rock the van back and forth until you could hear the sound of sloshing gasoline. Replacing the sending unit on that model would have required pulling the engine, so he learned to live with it. I guess he got tired of this nonsense and sold the thing to me. I got it home and figured out what the problem was in about an hour. The battery was hooked up backwards.