Here is another thought.
Using a clean fuel container, add a fuel line fron it via an in-line fuel filter to the lift pump. Check for air. If none, the problem lies in the lines to the tank. If they are present, then you have a problem with the lift pump and/or the filter/lines after it.
Relocate the fuel supply from the lift pump to the filter inlet (block the lift pump outlet to avoid spraying fuel everywhere)
If you have no problems, you have proven the problem is before the filter ie, the lift pump.
If you still have air problems, then the problem is the filter of connections to the IP.

__________________
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.