When was the last time this tractor ran? Strikes me that there are any number of disconnected/related/repeated threads running here & none are getting to the root of the problem. You need to systematically reduce the variables. Assuming that fuel is available through the system from tank through filter, to return and in sufficient volume to feed the injector pump, AND there are no sources of air leakage into the fuel feed before the Injector pump, the problem lies either in fuel solenoid/injector pump/or injectors. (I THINK, that you have established that you are getting good flow to the injector pump, so everything should be tight to prevent re-introduction of air before the injector pump. -- Not sure you filled the tank to eliminate a crack in the pick-up tube as possible air leak on the pre-injector pump side of the circuit.)
The next step should be loosen all the injector lines at the injectors & see how much fuel sprays out while cranking. If you get decent fuel spray from the injector lines, you have fuel to the injectors, but maybe not through them, so the injector pump & fuel cut-off is working (though whether to spec or not is for pumpguy or someone with a higher pay-grade than me to determine). If not, either your fuel cut-off or injector pump aren't working. Without knowing what has/hasn't been loosened/tightened/checked/set back to original/disconnected (since the tractor last ran), it's hard to pinpoint what is going on, since new questions/parts keep getting introduced (i.e. the new bleed valve question mentioned above). That's as far as my thinking goes. (if you do try to start without the fuel cutoff/be ready with a way to smother the air intake to kill it once you fire)
As far as air filter question above. The motor (any ICE motor) is nothing more than a giant air pump. To run, you need air (oxygen)/fuel (diesel/gas/nitro-methane/whatever)/and ignition/compression/heat to set off the explosion at the right time to keep the pump working. That moves air from intake side to exhaust side and the faster you move air, the more HP you produce since you need to introduce more fuel (bigger explosion) to keep the process running.
You raise an interesting question about Helene & non-working tractor ... namely, at what stage does it make sense to throw some money & have someone with more knowledge look at this instead of spinning wheels chasing solutions that don't work? Only you have that answer.
