There can be air entering from numerous points into the fuel system, this allows fuel to bleed off. First I would check the fuel pump, do you still have the manual lift.......
Do pressure and flow test on the fuel pump at the Schrader valve on the filter (FSS- fuel shutoff solenoid disconnected). Remove the tire valve core and hook a hose and pressure guage on it should see 4-6 psi when cranking. Then do a fuel quantity test same place, should see 1/3 pint in 10 seconds of engine cranking, route fuel to a suitable container with a hose.
If there is a problem with the lift pump and you are changing it, ensure the new pump comes with instructions as to correct installation procedure if it is the newer model pump with the almost straight arm.
Then visit.......
http://www.intellidog.com/dieselmann/idi2.htm look for air/leak points on the filter housing and base.....