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

air bag is my first guess go around all air fittings and find the leak ..... unhook the red trailer line to eliminate the trailer. ... check the "spaghetti " on the fire wall those are hard to hear for some reason

 

Probably now not the best time for spray bottle with soapy water but
Identify where most leaks are, Primary or secondary. Spray all quick connectors with soapy water and fix everything leaking.

 

50 PSI in 15 minutes you're definitely going to hear that leak.

 

I had a 07 that I also couldn’t find the air leak on.

It turned out to be the turbo actuator controller box that is mounted under the compressor.
It was leaking internally back into the turbo so you couldn’t hear it or see it bubbling with soapy water.

To see if this is your culprit, find the line from it that goes back to your air drier and block it off.Build up pressure and then turn engine of and see if your pressure drops.

By the way Cummins calls this their little money maker. It cost over 700.00 four years ago.

 

Likes been mentioned soapy water and a close ear and you’ll find it.

 

I eliminate air bags by dumping air suspension. If you have that option. Set tractor brakes that eliminates brake diaphrams. Someone already mentioned elimination of the trailer. Thats the most easy. Dont know your age, but i am slightly older. Grandchildren have great hearing! They say " its over here poppy" lol.