repair kit
http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=mercedesshop&cookieid=1CL0O6GWD1D30VCAWT&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1986&product=M2040-62843&application=000253332
Pump is 200-300+ US$ I got one for an 86 300E for $40.00 US at a salvage yard.
first run engine and watch pump to see if you can spot it
If you can have someone turn wheel back and forth that will accentuate a leak.
if you did not remove and replace it recently its most likley a front seal (lower left corner of the picture in the above repair kit link) where the shaft from the pully enters the pump boby.
best guess
best of luck
marty