RAF Techie 101.
You said
Vent lines contain air - hence the name
That of course is correct, but vent line can contain fuel, and that is the reason we test then. Leaking air is not a problem, leaking fuel is.
Once again, going back to Group Captain Hickman's statement, he said that it was difficult to test all seals/couplings in the vent lines, because not all were accessible.
So here is an idea; knowing that the real problem is not air leaks in vent lines, but fuel leaks, why do we not pressure test with fuel? Then we can
see were the leak is, and those couplings that are not "accessble" to the human ear will most likely exhibit runs or drips that can be traced.
As KeepItTidy put it "when garages test your brakes for leaks they look for hydraulic fluid". They do not pressurise with air and listen for a whistle.
DV