Hi!
There should be no air bubbles in the fuel lines in a correct set up fuel system! Investigate if there is a hole in the tank or in the fuel lines.
Do not! I repeat! Do not accept any air bubbles in the fuel lines!!!
Best tank to use, safest , with best fuel cap security of any available tanks is the Tettra tanks! The rubber expander that all of the other manufacturers use is no good in the long run! It will sooner or later leak.