However, the UAT is designed to opeate with a lot of air in it and still get no bubbles.

I have sucked a UAT comletely dry and flamed out my engine, the bag in the UAT will give good fuel flow until there is almost nothing left in it.

Most air bubbles I've seen that flame out engines were not related to teh UAT, it's usually a faulty connection point that's not properly wrapped in safety wire, or a bad connection at a festo or something like that.