The leaking valve covers are most likely the 'vent' covers ( the ones with the hoses connected ).
They come from the factory with MB RTV sealant, there is a thread on here about cleaning them up and installing with new RTV.
WARNING ! Those are aluminum bolts on the vent covers, DO NOT OVERTIGHTEN.
This is a VERY common problem, the rubber valve cover gaskets are usually not a problem, if the bolt torque is correct.