Im not certain about that engine but most of them rely on the valves in the supply pump to stop the flow back to the tank..
Install a clear line from the supply pump to the filter. & run it.. then when full. shut it off & wait till the next day to see if its still full or empty.
If its empty, u know the supply pump is allowing fuel to drain back..
U might be able to take it apart & check the valves for debris.??