I have a couple of suggestions which may or may not be helpful.
Usually fuel systems have a line which returns to the fuel tank. It is taking leakage from the injectors are sometimes air from filters.
I have seen fuel system designs which want this fuel return line to go to near the bottom of the fuel tank so its open end is always in the fuel. This ensures that as the fuel system cools down, as the engine cools down, if there is a suction created in this line it is sucking up fuel and not just air.
On the old tractors I am familiar with, this line inside the tank was metal and would vibrate off and then the operator was always having to bleed the system. There was no external leak to trigger some service but just constant air problems.
Buy some high quality rubber fuel line and bypass things like your fuel filter assembly. Install a cheap in line filter as temporary protection. You are looking for a time when suddenly you do not have to bleed one day.
Components which do not show a leak when the tractor is running can still have an air leak under low pressure conditions.
Feed the injection pump from a temporary fuel supply placed above the engine so you bypass the mechanical fuel pump.
Dave