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.htmlFWIW, 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.
