We had trouble on a John Deere 7830 after changing the fuel filters as the air suction issue was a poor gasket on the filter.
We have already ruled out any priming problems. The priming plunger is firm to push down with a jet of airless fuel coming from the tube after the filters and before the injection pump inlet.Fuel also comes out of the return line at the injection pump when the line is opened.It appears to be internal to the pump or with the two solenoids / valves on top of the injection pump. The motor turns smoothly.
Start with starting fluid and have the injector return lines removed and see if the fuel is coming out.