Here is the good news if it really is the FD. Apparently the Porsche guys have had the same issues and have gone a bit further with tinkering as detailed in the link above by Ivanerrol. Elsewhere, Deltacom has posted a link to a guy on eBay who sells diaphrams for the FDs to allow rebuilding. I contacted him yesterday to ask if he offered the diaphram for the FD on a 300SL and he indicated that he would have them in a month. Check the pics on Deltacoms post. Your diaphram may look like the one on the right which appears to be the logical result of years in a highly-volatile hydrocarbon environment compounded by more recent exposure to an oxygenated solvent (i.e. ethanol) to which nitrile butyl has poor resistance (I am guessing the likely composition of the original).

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1988 300E
1993 300SL
1994 540i

If I have a problem with my Mercedes, I go to MercedesShop. If I have a problem with my BMW, I go to BimmerNut.
If I have a probem with my Dell, I reboot it. I was an engineer for them for 10 years and we blamed everything on Microsoft.