the vent tube allows air to exit while you're filling the tank to avoid causing 'splash back', the vent hole in the cap is to allow for expansion/contraction as ambient temperature changes, which causes air movement up/down the filler
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
