07 9400i losing air pressure randomly at idle

thanks for all the info.... when I say I took it to some people to check these are the things they checked.

Turbo Actuator, actuator line and Solenoid
Fan Clutch input
All lines, leveling valves, Air Bags, brake Chambers, ABS solenoid and controllers where air lines feed into them.

The turbo actuator was tested doing the truck off/ key in on position computer test.

The only thing I have not changed valve wise it the valve under the drivers side, I love to call a hornets nest. There are like 30 air lines going into this thing. No Audible air leaks, however i guess there could be a blockage somewhere.

Today I got under the truck to check the air dryer and under the hood to check the compressor.

I loosened the main air line (metal) a little to ensure the compressor was actually pumping out air when the gauges where falling.... Air was flowing.

I disconnected the line on the bottom of the air dryer to make sure air was flowing thru air dryer. it was. opened drain valve on secondary tank and it had some water in it, as did the wet tank... drained them dry. Still no air building on gauges, no water out of the dryer when I disconnected the line.

Tried to release brakes with psi on gauges at 30, brakes would not release.

Got back under truck and disconnected large air line on primary tank, to allow faster escape of primary tank and to make sure the air was getting into the tank. air was flowing into the primary tank, and secondary tanks.

here is where it gets interesting. I plugged the air line back into the primary tank, and before I could climb out from under the truck and into the cab to read the gauges it was at 120psi, and the dryer purged. both tanks have been at 100 to 120 psi since, even when I shut the truck off.

Still no audible air leaks, and no idea what to do next.

 

must be missing something in your reply. this has already been do when visually and audibly checked everything for leaks. spent 4 hours crawling over in and under every inch of the air system, with truck on, off, and even with key in on position but engine off. Even passed full roadside dot inspection with no air leaks... I'm looking for other possible issues that might be something internal somewhere that isn't really that common.

truck only loses air at idle, not while driving or when the truck is off.

 

didn't mean to come off so rude... sorry about that... Just seems this is a strange one to figure out because we've tried all the standard checks and fixes and still have the problem....

Since last night the pressure hasn't fallen below 100 psi, even with truck off for 3 hours. the only thing i did was unplug the big airline going into the primary tank. I'm really starting to think there is a plugged line somewhere

 

Found out it was the connector on the bottom of the turbo actuator valve control box thingy under the compressor(not sure what it's called but I do know it cost $800). I noticed that the air line that fed into this looked kinked at the tractor protection valve, but wasn't leaking.... I spent the $0.65 a foot for a new line and started changing out the line. when I started to remove the old line I noticed the fitting was spinning and I almost dismissed this as it was supposed to do that... I looked closely and noticed it was threaded into the control valve so I checked to see if it would tighten up and it did. I tightened it down with a wrench and replaced the air line. So far since doing this (4 days) I've had no air lose, drove over 2100 miles in temps from 45 degrees to -1 degrees, elevation from sea level to 8700 feet.

also since tightening up this fitting, I have almost no water accumulation in my wet tank or secondary tank

Cummins ISX 2006 engine
2007 Int 9400i

Talked to Cummins and they told me this only pushes air once it hits 90psi, s that might be why I could not hear air leak, and it doesn't push air when the key is off.