air pressure loss?

It would help if you can figure out which systems each tank feeds. But what you are describing is commonly caused by a leaking airbag on the main or cab suspension. Soapy water sprayed around the base of the bags between the bladder and piston will find that.

 

Ditto to above. Also check for a leaking drain valve. Happened to me before

 

I would spray (with soapy water) every possible air line fitting and all airbags, using this method you are destined to find the leak.

 

Thinking the red is the primary for the brakes system. Kids bubbles and a squirt bottle work good for finding leaks. They can be anywhere-including behind dash-leveling valve-air seat valve-air horn valve-etc. etc. If you step on brakes and the red needle moves-start with the brake system but both are tyied together.

 

Guess I made the mistake of assuming no trailer... check the gladhand with the soapy water, too.

 

The Foot Brake Valve that's mounted on the Driver's side Firewall under the hood has a lot of Air Lines passing through the Firewall. It is common to have leaks there too. The Engine Fan Hub Seal could also could be leaking.

 

Leveling valves. Cab, truck chassis and trailer.