Cummins engines are prone to fuel drain back if the engine has some hours on it. Possible sources of air intrusion are: at the tachometer cable drive on top of the pump, and at the fuel filter head, if the filter is mounted directly on the back end of the pump. A third source possible is at the fuel line to pump fitting if your engine has a remote mounted fuel filter.
To test the tach cable connection, disconnect the tach cable at the pump and remove it. Start the engine and let it idle. Take a small amount of diesel fuel in a cup and pour the fuel over the connection where the tach cable hooks to the pump. The center of the shaft will be turning. Now observe the puddle of fuel. If the fuel disappears by being sucked into the pump, the tach drive seal is bad. This will allow air into the fuel system when the engine is running, and allow fuel to drain back when the engine is shut down.
If your fuel filter is mounted directly onto the rear end of the fuel pump, air leaks can occur between the filter head and the back of the pump. Because this area is under suction when the engine runs, no external fuel dripping is seen, but when the engine is off, air can leak in, allowing fuel to drain back. Using the correct size Allen wrench, tighten all the Allen head screws that mount the filter head and pulsation dampener (small rectangular piece next to filter head) to the rear of the fuel pump.
If you have a remote mounted fuel filter, (hose between pump and filter head) Remove the fuel suction hose from the back of the Cummins fuel pump. Now loosen the jam nut and remove the fitting that the suction hose was connected to. Note that the "suction fitting" has an "o ring " under the jam nut. Replace this "o ring." It is also a potential source of air leakage.
These three areas are where Cummins pumps are most prone to problems. As other members have posted, replace the suction line between the pump and tank, and inspect the connections for the fuel line to tank. Good luck.