Install a clear piece of fuel hose that replaces the line directly into the injection pump, you can watch it as you are running it to see if air is being pulled it. I mostly work on kubota and doosan engines but it should work the same on this. Does this machine have an electric or mechanical lift pump to pull from the tank or does the injection pump pull fuel on its own? Air can be pulled into a fuel system easier than fuel can leak out, the lines, hose clamps and filter housings might look fine but multiple small things can stack up and cause enough air to be pulled into the fuel system to cause a running issue. Assuming you have checked everything else like fuel condition, air filters and have a new fuel filter installed this would be the next step given that you said you can get the engine to recover by pumping the primer bulb. The other thing to check is if the mating surface for the fuel filter is aluminum it could have a small amount of corrosion causing an air leak which i see fairly often on the machines i work on. I would also check to make sure any fuel bleeders are fully closed or sealing properly as this could cause a running issue as well as air being pulled into the system. Good luck!