I don't know if you can replace just the sensor, or if you need to replace the entire reservoir:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41M10V00Q1&year=1986&make=MB&model=420-SEL-001&category=N&part=Brake+Reservoir