Before you go throught the PIA of changing out the modulator, ensure that this little cap is not the source of your vacuum leak : http://www.allpartsexpress.com/ShopByVehicle.epc?q=1984-MERCEDES--BENZ-300sd--/--L5--998cc--3.0l--Diesel--Fi--Turbo-Transmission&yearid=1984%40%401984&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1194038%40%40300SD++%2F+L5%2D2998cc+3%2E0L+DIESEL+FI++Turbo%40%40300SD&catid=Transmission%40%40Transmission&subcatid=Modulator+Valve+Cap@@Modulator+Valve+Cap&mode=PA

Because the modulator is not a plug-n-play part. Been there, done that and want to save you a little grief.

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Sam

84 300SD 350K+ miles ( Blue Belle )