I have been replacing vacuum lines on my 1990 300d 2.5turbo. Near the 4 way splitter near the pump, there is this little plastic filter-screen device inline. What is this called...I broke it and am going to try and find another.

I’ve seen those, I think it’s primarily to add a tiny bit of vacuum storage, and help prevent oil from migrating through the system and damaging stuff.

IIRC folks use fuel filters.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)