...When you located the coupler half way from comp to tank, do you remember the color of the air line? Also, mine are inside a large wire wrap/cover which I'll have to open...
On my truck the main line from the compressor to the storage tank is covered in steel braid, and it's a two piece line so the coupler joint isn't covered but it wasn't clearly visible because it's located high up near the top of the undercarriage and tucked in behind some other wire bundles so I had to feel along the line with my hand to find it and use a mirror to leak check it. As I recall I needed two 5/8" wrenches to tighten it and it was quite a chore to get the wrenches on the coupler and each of the nuts!
...I have soaped every connection and air bag under the truck with no luck...
Also check under the hood and inside the cab, the air seats and put your ear on the dash and listen for leaks in the places you can't soap.
...I borrowed a Holloween fog machine and plan on putting the truck in my friends plumbing truck building where it is quite and no air currents. Hopefully I'll find it then...
My truck has an external quick connect air fitting to fill the tires which is another place to check for leaks. I'm wondering if you could drain the main tank and then fill it through this external quick connect air fitting with some sort of coloring agent like they do to find oil leaks? Perhaps fill it with something that has a distinct odor!
...Right now the truck cab is sitting on the stops and air press is only 25 psi at the dash gauge...
To me this sounds like a leak in your air bags or the leveling valve! My truck would leak down to 0 psi on the dash gauge (which is another source for a leak) but my ride height didn't decrease by even 1/16"! Then if I pushed up or pulled down on the arm from the leveling valve I could hear the hiss of air being released from the air bags but as soon as I released the force on the arm the valve shut off and the hiss stopped and the new lower ride height was maintained! On the other hand my air bag hitch is connected to the main tank via a regulator valve and it did deflate as my main air tank leaked down.
...I wonder if the air comp. has to work harder with a leak like this?...
Yes it does! When I'm rolling down the road I notice that my air gauge cycles from 105 psi to 125 psi about once every 10 min to 15 min depending on how smooth the road is. So calling normal as about 5 cycles per hr, a leak of 20 psi per hr increases the compressor load by 1 cycle per hr or about 20% more than normal.
...Also, frustrating is the fact the attach points for the parking brake unit in the dash have all shattered (plastic) and I had to make some shims/brackets and use 2 min. epoxy just to firm everything up so I could use the valve. This has to be common and most certainly a safety issue. At least that's what I told Freightliner. No answer from them yet!!
Below is a pic of my parking brake handle, and the push/pull rod that activates the valve appears to be attached to a metal plate behind the plastic dash? Does your parking brake handle look like this?
This is my air gauge, since the truck has been parked since Mon afternoon and it has only lost about 35 psi in 5 days I'm declaring a "victory"in my air leak battle for now!