There is another phenomina that exists that would be very subtle. People can design fluid flow 'flip-flop' circuits, that will take two streams that merge into one outlet, and can make them draw from either side with the momentary application of a side pressure. Once started, the flow continues from the given side until it is purposely reversed. I can envision (but don't know how to design) a situation where there is a subtle difference that encourages that behavior.

Bill Schertz
KIS Cruiser # 4045

Bill;

I think that is a factor if there is sufficient flow that there are inertial and pressure drop effects.  Here we have a line that normally has very low air flow.  Maybe in the event of a fuel cap leak drawing high flow through the vent it could be an issue; but even in that case there are bigger problems than the vent imbalance.

Al