The filter neck is extended down into the tank to help prevent basically overfilling, but people will keep slowing adding fuel to the tank until no space is left and then put the sealed cap on and it will push the fuel up the vent hose in combination with the slight vacuum created on the vent tube by the running engine. Lincoln welders a few years back had issues with people doing that with the Kohler engine equipped units, and it was causing hydrolock while running.Shouldn't the roll over valve be stopping it? I can understand if is sloshing up the vent line before the valve closes but even that should be prevented.