DV asks:
knowing that the real problem is not air leaks in vent lines, but fuel leaks, why do we not pressure test with fuel?
The simple answer to your question is that pressurising a pipe and coupling with air will find a leak a lot sooner than using a fluid (water or fuel). In the latter case, it can require up to twice the pressure to be applied due to the higher viscosity of liquids [Source - BAES testing of Nimrod FRS couplings]. The last thing you want it to have fuel leaking as a result of a pressure testing. Therefore, in engineering and safety terms, using air to test the vent and other fuel pipes is correct practice. As pointed out, the solution is to stop fuel entering the vent system. This will be addressed, I believe, on the MRA4.
Non engineers on this forum should stop making such ill informed comments. We would not dare tell pilots how to fly (especially those who drive big Boeing ones).