Jeff, since I recently went through a lift pump failure I had the opportunity to open up my old one. The "O" ring that seals the pushbutton is a rather loose fit around the plastic shaft that acts as the piston for the manual pump. There is a shoulder on the shaft that seats on the "O" ring when the button is not depressed. That's why it only leaks when you pump it. It's hard to describe but the shoulder on the one piece plastic shaft is the actual part that pumps the fuel as you work the button. It's a separate "pump" and is not connected in any way to the actual metal piston that is driven off the camshaft. You can pull off the rubber cap, unscrew the nut and take the whole primer pump out without removing the lift pump. You should be able to find a matching "O" ring at the local hardware store, I did. If not I believe Cummins has them. Mine leaked just like yours but my real problem was the check valves in the lift pump itself started leaking and I lost prime. As long as yours is starting fine after sitting overnight or longer I doubt your minor leak is letting in air. If the pump starts giving you more problems such as losing prime, the latest Cummins P/N is 3936316.
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