Tractor Repair: fuel problem, david brown 990, air bubbles

Question
I have a David Brown 990 Selectamatic.  It is a great tractor and usual runs without problems (except for the hyd steering...)  Anyway, I recently had problems with air getting in to the fuel system.   After much research and troubleshooting (cleaning & replacing fuel lines, fuel filters, and the lift pump) the problem appears to be in the tank.  With the line connect to the tank, air bubbles are seen in the lift pump bowl.  I bypasses the direct connection with a hose in the top and left the cap off.  No bubbles and all runs well.   Now for the questions.  Is the problem the tank cap, the line inside the tank or ?  I am hoping to avoid cutting open the tank to find the line inside is fine.  

Answer
I don't think the cap is the problem -- Without seeing the inside of the tank I can't say for sure that the line inside is your problem --- However -- if the fuel system is sealed and you bypass the tank -- The only part of the tank that is part of the sealed system is the line -- if the line is always submerged in fuel - then a crack in the line in an area were the line is not submerged in fuel is the only place I can see air entering the system.