Tank vent partially or fully plugged, bugs or other debris in vent line?
If the fuel pickup tube has a screen it may be partially blocked by debris, if you can get it out it is worth checking
Assuming the filters are between the tank and the pump either may be sucking air, hard to find. An electric fuel pump ahead of the filters will pressurize the system and possibly show where the leak is. Perhaps something like this?
The Diesel Page, Electric Lift Pump and Racor Filter installation
Also, an obstruction or crimped fuel line may make the fuel pump pull a higher than normal vacuum in the system causing an air leak which can be hard to find. The diesels I had did not have the hand priming lever but I have read the engine has to be in a certain spot for the lever to work? Let us know what you find.