You need to get to Tractorsmart.com and get the parts break down for your tractor. It will show what everything is. If your tractor starts and runs, you don't need to keep cracking injectors to bleed air. Once it is up and running, your good there. If you are not getting fuel to the injector pump, many times it is the fuel delivery pump. I have no idea if your tractor een has one as many Kubotas have a gravity feed system if I am not mistaken. You may have a air bleed valve like my Kubota. If I leave it open, it will act up much like you describe. It is located very close to the injector pump. It will run with this valve open, but it does need to be closed once the tractor gets going. We had a problem like yours on a Kubota L4840. There was a blockage in the fuel delivery stemming from the tank outlet. Seems a bit of mud got into the diesel can which in turn ended up in the fuel tank blocking most of the fuel flow. It was obvious by looking at the clear fuel filter bowl that there was insufficient diesel in the filter system. If you can see your filter and the housing is not full all the time, you have a delivery problem.