MD11C air in fuel line

I'm getting air in the fuel line, or maybe I'm pulling a vacuum. Either way, bleeding the fuel system is the quick fix, but problem reoccurs, usually several day-trips later totally 2-4 hours of engine time. The problems doesn't appear to be related to engine speed or load. It even occurred while it was idling at 1000 rpms at the dock. It happened before, and now after, I changed all the filters (two CAVs, lift pump screen and main filter cartridge.) The only common thread is that the engine is at operating temperature when it happens.All the fittings are snug. New O-rings on the high pressure lines at the pump. The main filter was assembled on the bench to ensure that the square O-ring didn't twist. Two fuel tanks "T" together to feed the lift pump. Bottom of tanks are higher than the engine. Even if the lift pump fails gravity feed should be enough (?). Fuel lines were replaced about 6 years ago and are in pristine condition. Lift pump diaphragm is probably 50 engine hours old. However the rubber hose, the one with the banjo fittings is probably 1977 vintage, but there is no evidence of diesel on the outside of the engine (leaks.) Any ideas?

What i did was plug in fuel from a jerry can at different stages along the fuel entry to the engine starting from almost direct to the fuel pump and working my way to the tank...i ran the engine for 10 mins at each testing point and then tried to bleed the fuel immediately after to see if air bubbles come out...in my case it ended up being the steel tank nozzle...the problem only emerged when the fuel got to a lower level than the sea water strainer and filter...this only occurs when the tank is low... although a different engine to yours it may offer some insight...
Ed