Cummins N14 start problem

The check valve is in the brass fitting that connects to your fuel line that goes to the filter.
When looking at ECM it is located at the top left. You do not have to remove the ECM.

 

All great points, most drain back issues I've seen are fuel pump. That's why I would start with a line in a bucket. Work backwards. Those cooler plates will suck air.

Generic steps are making sure tanks vents are not plugged. Visually inspect fuel lines. Fabric braided lines fail around 7-8 years. The will suck air before they leak fuel.

 

I checked the tank vents, all good. Fuel pump was replaced half a year ago. Do you think it could be failing so soon?

 

Cummins part or aftermarket?

Seriously, take a line from pump straight to a 5gal bucket and test. If problem goes away, your issue is before pump. If your problem still exists. Its pump or after. Simple diagnostics.

 

Follow what @jamespmack said first. I've also seen the ECM cooler plate get debris packed between it and the block and corrode and do just like James mentioned about fuel lines sucking air before leaking, I've seen the same thing on cooler plates due to all the dirt/sand/leaves etc packed in behind it.