Injector pump mystery

[Sea tractor]

I have a Kubota 60 hp (V 2403) that runs perfectly ………if the first injector fuel line is loosened. Air(foam) and a small amount of diesel leaks from the nut (about 1 drop every few seconds). If I tighten the fuel pipe the engine runs for about 20 seconds, hunts then dies. Obviously air is entering the fuel system. I replaced all the fuel lines with clear hose to identify the source. No air bubbles all the way to the injection pump. Opening the bleed valve causes air bubbles to emerge from the pump and travel up to the low pressure (bleed off) hose to the injector. I notice where the high pressure fuel line enters the injection pump there is a small amount of play and the nut can be rotated slightly but is not loose if that makes sense. I think this is where my problem is. I just can’t fathom how air is able enter this hp fuel line without highly pressurized diesel pissing out of the pump. I don’t think I have the temprement or skill to tear apart the injection pump so may have to consult a professional. The other option is to live with bleeding off the air and catching the drip.
As I mentioned the engine runs perfectly in this condition. Has anyone run into an issue like this? Please see attached photos

 

[D2Cat]

Are you sure your return lines are open?

 

[Sea tractor]

Are you sure your return lines are open?

Yes, I’m sure the return is open because I have temporarily replaced the fuel lines and return with new hose run to a jerry can. The return is long enough to go lower than the can and back up to the mSomeone else mentioned the return but I don’t understand why. How would a blocked return line introduce air into the fuel system?. Many thanks for your response

 

[Kenonf]

Hi,
remove injector lines and check injector pump delivery amounts, if they are similar then remove injectors. Using the pipes and injectors re attach to the pump to check if you have sufficient pressure to pop open injector and allow a spray of fluid out of the injector that you can observe. you can hold cardboard behind injector to see spray pattern, do not use finger to check spray, it is at a high psi and will easily penetrate skin sending you to hospital.