Check the bronze filter under the plate in the reservoir. It is sandwiched between the drain baffle plate and the bottom of the res..you have to remove the plate to see it and many do not even know it is there b/c it is hidden under the baffle.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42B7142NRM&year=1968&make=MB&model=280-S-001&category=M&part=P%2FS+Filter

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A Dalton