When you install a two-tank kit in one of these Mercedes, you need to re-route the fuels lines at the diesel filters, as well as plug the banjo bolt on top the spin-on fuel filter. Did you do this originally? If so, did you make sure to get it back the way it was?
I agree, it sounds like you have a bunch of air in the lines...you'll have to pump a lot to draw the fuel up to the IP, or use a mityvac on the return line to pull fuel to the pumps.
If you need local help in socal with your VO system (or just mechanic help), a guy named JeffnLisa on the frybrid
forums installs VO kits, his email is dandsins (at) yahoo.com