2016 Cascadia, Have an air leak nobody can find.

Did anyone check behind the dash? Airleaks at one of those thin airlines can be very quiet. Maybe at a valve, as well.

 

Lines on top of the trans can be a pain to check.

 

Listen at the breather. Could be a leaking splitter/range piston leaking into the transmission housing.

 

If it fails an air brake test what’s the chance it’s a brake chamber. Leaking down after 6 hours isn’t all that unusual but if you hold the brake and it leaks down fast I would be checking your brake chambers

 

Sleeper leveling valve or airbags...?
Suspension leveling valve
ABS valves
Tractor protection valve
Line from compressor to governor
Related lines and fitting...

 

gg, 6 hrs?
Google says..
"After the initial pressure drop, the loss rate for single vehicles should be no more than 3 psi in one minute and no more than 4 psi in one minute for combination vehicles"

So, 120 psi : 3 per minute = 40 minutes till the system is legally empty.

6 hrs x 60 min = 360 min : 40 = 9 -1 = your system is 8 times more than legal)

 

If the audio is not present it may be in older.. inside of Air dryer, compressor, parked chamber, transmission.

 

all your numbers are for the AIR BRAKE test and have nothing to do with how much air a truck should lose when turned off parked

 

I’ve mentioned this in many of these type of posts. 99% of mysterious air loss is through the air shifted manual transmission. They will be hard to detect and can be slow and quiet. Install a shutoff valve on the air supply line for the trans and let sit to see if air leak stops. It was/is not uncommon to have to leave a shift selector button in dir or od when parked to keep from leaking.