You are sucking air, and by the looks of the prefilter having a small air bubble, I can bet that the problem will be after the prefilter. I have experienced a cracked prefilter where the air leak would allow air into the fuel line to the lift pump, but not into the prefilter. This was exacerbated by the prefilter being partially clogged.
I doubt that you have a leak from the main filter, you would have a noticable fuel leakage when at idle.

Replace the prefilter and the small rubber hose which connects it to the lift pump and report back to the Forum.

Tony
The Wizard of Oz.

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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.