I couldn't find clear small fuel filters at the auto parts store but found a clear 1/4" lawn mower gas filter at the hardware store. With a little dish soap in the hose, I was able to force on a piece of vacuum tubing to each end which will also fit the brown vacuum tubing in the car.
The W116 300SD had no OE vacuum filters that I can see, unless a PO removed them, but the lines look original. Since it has the dual diaphragm vacuum pump with separate lines to the brakes and everything else, a person would also need a filter in the large brake vacuum hose to detect a failure in the pump vacuum diaphragm even if a filter was put in the 'everything else' line.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13