One major problem with Mercedes Diesels is that you really need German Hose Clamps on any conection.Don't buy the hose clamps that bite into fuel lines.
With your German clamps now you can tighten the hoses so they don't spin on a conection.I always trim the ends of a hose,when I replace a filter.
Check your tank hoses too.I mean a fuel line may not leak,but suck air.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt