Hector,

If you orient the UAT as Andy indicated, then when you refuel the model thru the UAT, you will displace the air bubble (with fuel) out of the UAT and thru the tank system. If the UAT is vertical and you refuel thru the UAT, you'll be starting with a bubble in the UAT already as you've noticed (under the screw-on cap). I don't think it is necessary to have all the air out of the UAT, but I'd say that less is better.

I orient mine as Andy states, whether it is a UAT, or a similar tank with a centered fuel filter...