I'll go along with Donnie on this one, however the unit your talking about is most likely the Differential pressure regulator (DPR) or MB speak (EHA)
If it's leaking in the back it needs replacement. In the front you could get lucky with O-rings.
On this car I believe the Pressure Regulator is part/internal of the Fuel Distributor, but serviceable.
Replacing it might solve your problems, you should still have a pro
take a look at it.
Someone that knows what theyr doing will save you $$$$ in the long run!