I am a Fuso tech and 40% of the 2004 Fuso's have electrical lift pumps in the tanks. I wasn't given a VIN so I asked in case you had one of the exception vehicle.
You still need a fuel pressure reading. The fuel gauges offer both a vacuum and a pressure, depending on which side of the pump you are on. If you have a low vacuum on the back side of the pump, (like on the fuel filter showing low suction on the port) then there is an issue on the suction side of the injection pump causing the low fuel pressure to go back out. You already replaced the injection pump so my concern is if there is an issue with the fuel filter assemply or a line collapsing. These engines are notorious for line collapsing just off the tank where the lines run up along the rail at the back of the cab from them being exposed to the elements and softening just before it goes into the filter. I usually will rule that out by dropping the line off the suction of the filter housing then run from a bucket to the filter housing. I will then run straight to the injection pump to rule out the filter housing and make sure the issue is on the front side of the pump and not the suction side. That needs to be narrowed down first.
I'm going to presume the overflow valve was changed with the injection pump and wasn't reused? If that overflow valve is sticking in full open, it will have the fuel just dump off and not allow the rail to build pressure.
From what you are describing though, I will presume the issue is at the suction with wither a collapsing fuel line (very, very, very common on these trucks between the fuel filter housing and the first joint to the hard plastic line where the soft rubber is) or it is in the filter housing.
If you find it to be the fuel filter housing, it is usually in the head in the round section. If you drop the filter, you will see a little valve up in there. It is usually gummed up with debris or plastic pieces from the old desiccant filters. You can sometimes blow it out by removing the 2 lines, then blowing backwards from the pump side of the filter. If you blow from the tank side, you will lodge it more. There are times I will even remove it and drop it in a small container of mineral spirits for the night and reinstall it in the morning.