I run a 1/4" fitting and vacuum tube from the top of the filler, to drain under the engine bay (for inadvertent spills at the filler), and use a roll-over valve/vent at the top of each tank, with a hose turned down to drain (again) under the engine bay. Nothing fancy, and have only seen the vent tube in use one time when I overfilled the tank (transferring fuel from tank to tank).