The 90 is usually a good engine and doesn't generally suffer as many fuel system issues as say a 125 (both being tddi).
The description you give of needing to keep 3/4 a tank reminds me of my old transit van!
If there are no leaks in the supply side from the tank and the filter head is okay with no vacuum leaks, it does seem like you have checked this or in this case is unlikely.
My van had a crack in the pipe that goes from the in tank pick up and reservoir bowl at the bottom of the fuel sender unit up to the top of the unit, when the fuel level was above this, no issue, when it was below, symptoms as you describe with the fuel running back down to the tank.
If there is no fault in the supply side or in tank pick up, then it could the low pressure side of the vp30 diesel pump being unable to provide enough vacuum to pull the fuel through.
Around less than 1/8 full it's fairly easy to drop the tank and stay dry, I done mine on the floor, replacing the whole sender unit. let me know if you want a quick run through.