Invisible air leak

Check your fan clutch, I seem to remember running into that once.

 

try pouring food coloring in the line coming out of the top of your compressor and the line coming out from your airdryer. then just look around and see if it shows up anywhere. I have also used snap line chalk.

 

If you really want help, please answer all the questions that everyone is asking. You are still not explaining the actual problem. Do both gauges drop? Is the governor cutting in and out correctly? Is the air dryer purging correctly?

There were several questions in my post and you only answered 1. Once you answer them all, I'll try to help you further, if not, I'm done here. We can't rule things out when questions are ignored.

 

If the air tanks have never been drained, they could have so little reserve capacity that any small leak would drain it down that fast. I like grapeape's idea about supplying shop air.

 

Invisible air? Can you normally see air in Illinois? I'd go with the Ape and a hand pump up sprayer with soapy water often works well. Braided air lines can loose a lot of air, working like a sprinkler hose, and not be heard. Multiple small leaks can do the same thing.

 

Had to have the firewall pass thru replaced on my 2007 Volvo, been several years ago, but I almost want to say my leak there could be heard.

 

I have a hard time believing that you can't hear a leak with that much air being lost. Unless you've got hundreds or thousands of very small leaks all over the truck. Now I'm wondering, you stated that both gauges drop but is the air actually bleeding off or is it simply the gauges dropping?