Isn't the fuel tank vent only one way? I thought it had a check valve for out only "Vent". If not a Dirt Bike tank vent check would solve that for cheap. If this is the case.
Can't be 1-way. Need to let air into the tank so it doesn't create a vacuum.
The dirtbike vent caps are a gravity check valve. They stay open until they're tipped too far. Keep gas from spilling but let's air in when it's upright.