Shark,
For air to be in the system, the fuel must be leaking out, or back to the tank.
If there is no fuel leakage from the engine, then the fuel must be going back to the tank.
The lift pump (including primer pump) has non-return valves as part of it. If these are working correctly, no fuel should leak back to the tank from upstream of the lift pump. So, if there were a leak on the tank side of the lift pump, your engine should start and run for a short time then stop when the air hits the IP.
As this is not your symptom, then the non-return valves in the lift pump must be allowing the fuel to return to the tank. This may be because of O-ring issues, which cause leaks from the Primer Pump (replace with the new style) or because the lift pump itself needs an overhaul.

Even if the non-return valves are faulty, there should be no problem IF there is no way air can get into your fuel system upstream of the lift pump. You must have an air leak into the fuel system.
If you repair the lift pump, you may soon find the leak, as lift pump efficiency will improve markedly.

__________________
Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.