Well, lets apply a little common sense to this. We hear the
story repeated often of guys topping off their AD with gas
that was in the ground at 55°, then parking it in the sun
without burning off any of it and the fuel puking all over the
side of he truck and running board. That would indicate that
it is vented in the positive direction. On the other hand we
know that a plugged or un-vented gas cap will, in short order
starve the engine of fuel, that would indicate that it must be
vented in the negative direction also. So it would appear
that the caps would have to be vented in both directions.
Which brings us full circle and we're back to fuel percolation
when the engine is shut off hot.
dg
