"C J Campbell" wrote
Okay, that much I knew, but why doesn't the lower tank just empty itself?
Does it need the pressure from the higher tank in order to start siphoning
out the fuel vent?

I think the lower tank does empty itself... but just until it gets to the
level of the fuel vent.
In the BOTH setting, they'll both empty to the level of their respective
vents.
Keep in mind that the right tank is vented to the left tank, and the left
tank is vented overboard, so if possible, it might help to make the right
tank the lower one.

Eric