L3200 dies and won't restart

Spreading gravel with box blade, tractor died and restarted ok, died and restarted and then died and well...died. We bled lines and got it going again for two rounds about the driveway and it died again. There was air in the lines when we bled it and now cannot figure out where that air may be coming from since it died again. Any ideas anyone? No leaking hoses that we can find, might there be a crack in the fuel pump or something? Box blade is not completely down when tractor dies as it is spreading gravel. Might there be a switch that keeps it from coming back on if it isn't completely down, a safety or something?