To add to what Lee said, the 4104 fuel pump is a mechanical suction pump mounted at the engine so the complete supply line up to the pump is under suction, only from the pump on is it a pressure system. This is the reason it is hell if you have a leak before the pump!! I think the pump pressure is around 35 psi but don't hold me to that number.
Look at some of the Bedford engines. Those are pretty much a DD engine, just rebadged. That is NOT to say that they are a 53, 71 or 92 series for you purists. Bedford was bought out by DD a longtime ago.
Another way to look at it is to look at a Jag XK injection pump.... it was mechanical and didn't self prime either.