Thanks for the post Paul!

By check valve do you mean the small valve on the side of the on/off Pressure Switch, or the actual Check Valve in the tank inlet (which the main Supply Line attaches to)?

You can see a full breakdown of this unit on our site by clicking on the below link:

www.mastertoolrepair.com/air-compressor-...-232177-p-16765.html

- MTR

Most likely your Check Valve is simply defective, yes. This is a one-way valve that typically gets clogged with debris, or the ball and spring falls out into the tank not allowing it to seal completely. You can try cleaning it first, but it may require simply replacing the Check Valve.

Here's an easy way to test the Check Valve:

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

- MTR