I found out after much head scratching and cussing that you do need a return line from the Weber- not having one hooked up caused severe dieseling when I had both a DGEC (counterfeit Weber) and a DGEV which it has on there now. (2.6 Litre.) Some of the regulars on this board know what I went through in the course of figuring that out.

The Weber DGV fuel bowl is vented to the atmosphere. If you go to the "2nd Gen electric fuel pump install" thread in the tuning and smog section you can see photos of the plumbing on my truck. I used the factory supply and return lines (electric pump in back). With a Holley 12-804 regulator set to fully open my fuel pressure is about 2.9 lbs. which is just about perfect.