These are all good points to watch out for. I had a UAT cap that would not seal properly under full power and was sucking in a small amount of air. Found two problems - the feed line to the fuel pump was too small (upgraded to medium Tygon, be sure to wire wrap or wire tie these connections!) as the UAT was collapsing, and the cap just would not seal tightly. I used a couple wraps of teflon tape around the neck and it sealed perfectly afterwards.

Barry