There's alot of things on top of the pump it could be. Can you narrow it down to governor controls or the distributor end with the 4 high pressure lines? If its running good I could only assume the fuel kill solenoid on top of the distributor head is loose or a return side leak or an air bleed. I would clean the pump with ether and compressed air till its bone dry and start it up and observe it. Its a bosch ve pump. There's plenty of descriptive pictures online so you can identify what feature is leaking.
It might also be worth checking the seals and gaskets around the top of the pump. Sometimes they can just be worn out or not seated properly.
I've had a similar issue with my Case 1840. The problem turned out to be a small crack in the fuel line that was hard to see. Make sure to inspect all lines thoroughly.