The filler neck has two hoses Tee'd into it one large lower that feeds to the tank (guess its to allow air out of the tank while filling it) and a smaller one that allows "vapour" into the small hidden tank then back to the actual tank and to the breather that I want to use... So the way I see it is if I put the breather straight to the tank return then the small tank will be venting via the filler neck... downside is any vapour that collects as liquid will drain out of the small tank and onto the ground