What will cause my float bowls to go dry

RRR,
I never said the fuel in the line drains back into the tank, but I should have been more clear: the fuel in the fuel bowl cannot leak back through the pump into the tank.
The n/seat is higher than the fuel level in the bowl. Iffff fuel was to leak back into the tank, the fuel has to be raised [ go uphill ] to get past the seat in the n/s. The moment the fuel level drops [ only has to be a fraction of an inch ], the n/seat opens & is subject to an air break coming from the bowl vent.