Kioti DS3510

Tractor experiences intermittent no-start condition after running normally for several hours. Fuel gauge non-functional. Priming pump delivers fuel to filter but appears unable to pressurize injection pump adequately. Bleeding injection pump top nuts yields no fuel or air. Issue persists despite fuel availability at injection pump inlet. Occurs at 380 operating hours.

Here's the story I found out a few days ago that my fuel gauge isn't working. Ran out of fuel. Filled it up, bled the fuel lines by cracking the bolt on the fuel injection pump and it cranked right up. Next morning it cranked right up and I ran it again for several hours with no problem. Stopped to wash off the tires and put it up. Finished washing the tires and it it wouldn't start again. Not out of fuel this time, but I figured, there might be a little air bubble still in the line. Cracked the bolt and it started again. Shut it off. An hour later tried to start it to move it and it wouldn't start. Cracking the bolt did not get it to start again. There is fuel coming out of the bolt, no air. As recommended in the factory service manual I opened 2 nuts on the top of the fuel injector pump to bleed any air out up there. But neither fuel nor air is coming out of the top of the injector pump. My question is, is this a bad injector pump? Is is air locked? Why would it have run for several hours that day if the pump had air in it was air locked? That doesn't make sense to me. The priming pump is delivering fuel to the filter port, but is it possible that it's not delivering enough pressure to get that fuel in to the injection pump? Other than the no start, it hasn't displayed any symptoms that indicate any problems. No hard starting or hard running, no issues with acceleration, no funny noises. I'm just trying to find any potential problems I'm missing that aren't a $1500 injection pump. The tractor (and pump) has 380hrs on it. I'd hate to think that it's a complete failure of the fuel injection pump.