09 T660 Air leaks as soon as I shut the engine off.

If bags are dumped you have leaks in secondary(FRONT) part of the air system. From the air tank one line is going to the front brake thru brake valve. This part leaking really rare and should be audible.
The other line from same tank is going to the pressure protection manifold (should close valve if pressure drops down to ~20 psi) and from manifold many smaller lines to rear suspension bags, cab suspension, gear box air actuator, rears lock actuator, turbo actuator, cab seats, cab dust blower, fifth wheel slide and release.
In my truck same huge non audible leak was on small line under floor mat to passenger seat.
You can replace leaking turbo actuator yourself, not biggy.

 

On top of the transmission on some of those, there's a 90 degree swivel air fitting... those are notorious for taking a ####. You have to remove the skirts from under the passenger side door to get to it.
I've never seen a pneumatic VGT on those... don't think you need to worry about it being the actuator for the turbo.
As for possibilities, they're endless. It would help to know if it's the primary draining first, the secondary draining first, or if they drain uniformly. If it's the primary, that would actually be easier to chase down.
If it's not leaking with the air bags dumped, then I'd inspect those airbags, as well as the supply line to the leveling valve which inflates those airbags.
How often has your truck been purging while it runs? Because if you're frequently going into purge cycle as it runs, you're looking to burn out your compressor, because it's not spending enough time in an unloaded state to cool down.

 

I have a 10 Kenworth t660. And as a fellow owner operator I could tell you take it to the mechanic you're wasting time trying to figure it out yourself you make too much money driving the truck to worry about trying to fix things yourself. You have your profession and they have theirs. But I got to tell you mine's been in the mechanic shop and had just about every air-powered part of the truck replaced that broke and it every time it comes back with the exact same air leak overnight all the way to zero and there's no sound and nobody can fix it I've sprayed soap on the lines we've done everything replace brakes airbags air leveler valves then valves on the dash, there's just nothing left to replace and it's still loses all its air overnight all the way to zero without making a sound.

 

Did you read or try my suggestion from almost 6 years ago?

 

Well I'm sorry I can't find it I'm trying to go through the threads. And yes I always get my sleeper bags replaced in the shocks and the leveler broke off and we fixed that. Like I said I didn't change these parts just to change them. They actually broke so we thought we found the air leak and we'd replace it and I'd still have the air leak. One by one other things did, but never resolves the original issue. And the truck didn't do that when I first bought it used but I've had it 10 years and I think this started about 5 years ago. So all I can think is that it's the airline somewhere inside the conduit running along the frame must be chafed.

 

It’s 9 or so posts above this one…..