Thanks for the post!

What you could have is a defective tank Check Valve. If the unit pumps air just fine, but leaks air after the unit fills and shuts off, your Check Valve is not sealing the air off in the tank; most likely it just needs to be cleaned or replaced.

Try the below test to be sure:

1)Let your unit pump up. 2)Turn it off. 3)Find the supply line from the pump to the tank. 4)Remove the supply line from the pump head. 5a)If you hear air coming from the tank you have a defective check valve.

You can click on the below link to view a technical document as well as a short video:
www.mastertoolrepair.com/how-to-test-check-valve-p-22524.html