I too can state the obvious; The MAIN purpose is to allow air into the tank to stop it collapsing as the fuel is drawn off. BUT... when there is vapour pressure in the tank, does it lift the ball and close the valve? (the 3/4 steel ball weight, would hold back a bit of pressure)
Many carriers were built with the sealed cap, and vent tubes.
Were problems with these the reason for the "valved" caps/ non vented tank design?
Which design came first.
Was the vent pipe design o.k. for northern climates, but unsuitable for the desert theatre?
So many unanswered questions
