MacDaddy,
1. What is a long period of time?
2. Is your gas tank behind the front/driver's bench seat?
3. What do you mean by "gas runs back to the tank"? Do you mean the gas line goes dry (drains/leaks dry)?
It most likely does not drain into the tank unless it is siphoning back into the tank (panel/suburban). If it is siphoning into the tank, there might be an air leak near the carburetor?
Maybe someone else will have better advice.
Good luck,
Tim
