07 9400i losing air pressure randomly at idle

I have a 9400i with Cummins ISX. While idling the truck will build pressure to 120, purge and remain at 120. randomly the truck loses pressure down to 30 psi in each tank.

I have been chasing this for a couple of weeks now. No one I take the truck to can find any leaks anywhere.

Truck off, no audible air leaks, and truck running no audible air leaks.

Compressor is only 2 years old, air governor is only 1 year old, air dryer is only 4 months old.

At highway speeds air stays between 100 and 120 psi.

anyone have any ideas or suggestions.

Thanks

 

just replaced fan clutch 3 days ago. still happening

 

tried checking it this way but no audible leaks. what should i check in/on dryer? this started after we replaced the dryer with a brand new one

 

spitter? you mean the purge valve, heater wire is connected, i will double check everything again.... please keep giving me ideas, anything helps in this hunt.... thanks.....