OK Paul - I just thought that the fuel in the pipe to the tank may have drained back. So, it looks as if the air is being drawn into the system somewhere. With diesel engines, the fuel supply is under negative pressure, so air bleeds in, and fuel doesn't bleed out - get my drift? So, it could be the filter unit being incorrectly assembled, or the primer pump diaphragm has become porous (very common on earlier Fords). Presumably the pipe runs from / to the filter havent been interchanged? Any sign of poor fuel pipe runs from the tank? Try pumping the primer button as a friend cranks the engine.
John, April 2009