Impossible to find air leak

It's both tanks. The Horton, seat, transmission air and some other items drain the front (left) tank but the brakes drain both tanks when you work the foot brake valve.

 

Doesn't have an air drier!

 

Already tried that. I did find the check valve from the wet tank to the compressor was leaking back, I replaced it and it's ok now, but still got that air leak.

 

No, just an air seat and Airglide 100 air suspension. The airbags stay up for a week.

 

You can isolate the air start, there's a valve to turn off the air starter tank supply. So definitely not that.
It's going to be well over 100 degF today so I'll go down early and check out the air tanks for a pin hole leak. I've got some specialised plumbers spray which is like soapy water but sticks to whatever you're spraying. They use it for household gas leaks. Imported from USA btw.

 

I've had that happen myself. But the airbags stay up for a week and the system drains in a couple of hours. When the truck is empty (no load on, just the trailers) the airbag gauge stays on about 20 psi and it will be like that for days. I've sprayed around the Hadley ride height valves and no leaks. I dumped the airbags for a night, no difference.
When you dump the bags there is still air going to the ride height valves, but I can't find a leak. Next thing to do is to completely isolate the air bags from the air tanks.