Air leaks only when compressor running

I have a brand new IR 5 hp 80 gal 2 stage unit. Air leaks from the pop-off valve that comes off of the pressure switch, but only when the unit is running. There are 3 pop-off valves on this unit, so I moved them around and the only place it ever leaks is from the one at the pressure switch and will not leak any air when the compressor is not running. I would like to know the issue before I return to where I bought it so I know what to expect and how to proceed. Rob, Nebraska