In my 300SD, the diaphragm (monovalve) was indeed the problem.
If your car is setup like the w126 sedan models, the valve is near the firewall, slightly toward the driver's side from the battery.
If you look at that area, you should find a ~2 inch square cover with a connector at one edge, and a round bit sticking up in the middle, about 1/2" diameter. That little bit in the middle is the top of the diaphragm that you are looking for.
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2830II46329406L13C&year=1987&make=MB&model=560-SL-001&category=R&part=Mono+Valve+Repair+Kit