I’ve got an 80 gal upright Black Max at the shop. It has a 1/4” line going to the pressure switch from the check valve of the tank inlet where the compressor pumps to. The small valve has an air leak all the time now. Everything still works but it bleeds down the tank overnight. I’m not sure if this valve should have pressure on it after the compressor shuts off and the check valve stops the air from the top of the tank. I’m wondering if the check valve is what’s bad and it’s letting too much pressure back to the 1/4” line and the pressure switch. I’ll post a picture of the pressure switch and line. This little valve is called an unloader valve.

The check valve is most likely the culprit. You can apply all sorts of logic to what should be happening and what's at fault but in the end it will be the check valve 99 times out of 100.

Here you go - the first paragraph says it all !!

https://mastertoolrepair.com/air-compressor-parts-check-valves-c-25_4.html

FWIW, you need the correct replacement part. In my case the Campbell Hausfield part had long been discontinued but an Ebay search found reproduction one's made by a guy in the U.S.. Price was reasonable. I swapped that a few years back and it solved the problem and has worked flawlessly ever since.

Yeah, the unloader valve bleeds down the pressure on the discharge side of the compressor when the motor shuts off.. Without it, the compressor could not restart against the tank back pressure. The check valve prevents the tank from bleeding down through the open unloader valve during the off cycle.

I think you are correct. I’ve been looking at check valves on line and it seems to be the most likely culprit. I could not see how that small unloader valve could handle much air pressure.

It's the check valve ,

Yup. Replaced the check valve and no more leak.