On dual tanks the front tank is filling up rear through return line how do you fix it?

- Bad check valve in the return line.

- Return line is higher then the tank.

- There is a leak near the tank causing a vacuum which in turn is pulling the fuel from the rear tank into the front tank through the return line.

Solutions

1. Check the return line for leaks and repair any found.

2. Ensure that the return line is not higher than the tank.

3. Replace the check valve in the return line.

4. Ensure the fuel cap is on tight and the filler neck is not leaking air allowing the tank to go into a vacuum.

5. If problem persists, check for a bad fuel selector valve.