Low air alarm

Kinda what it sounds like to me, the sensor

 

The lines to it could be plugged and holding a vacuum, but that's unlikely because it does it under pressure. I'd pull the sensor and see if it needs cleaning or replacing.
On second thought, you don't shortcut on safety devices: replace it.

 

Fairly common issue with Detroit's
Put a new element and O ring kit in the Dryer

 

Learn something every day here. Let me try to think this one out from flaccid memory. The dryer is getting plugged and the air is showing low because the dryer is between...ah, don't tell me, I'll get it...the pressure tanks and the pressure sensor.
But... the brake system has to be after the dryer too, so there would be less pressure there too. But the pressure gauge is connected before the dryer (tank pressure) only on detroits?

 

Also, the pressure alarm will sound before there is a problem to the breaks, Thus, you think that the breaks are fine.

 

like they said check the sensor and the wirering going to it

 

For future reference, the 2nd needle in your air psi guage is not the pressure going to the trailer it is your reserve air

 

Looks like the guys have your problem solved. Different truck, but the alarm in my '96 W900 about drove me nuts. For some reason it would just go off, seem to have different causes. The alarm was in the printed circuit panel in the "overhead" dash, and one buzzer was for several jobs, low air, high temp, check engine, air suspension dump valve, diff lock, etc. You had to look at the printed circuit lights to see why the buzzer was going. Sometimes nothing was lit up, I would pull over and flip all the related switches, which sometimes helped. Replacing the actual buzzer, and the printed circuit panel several times seemed to help for a while. Never did find out the cause, but I think it was in the wiring, as replacing stuff helped. When I sold that truck, I gave the new owner a box of the p.c. panels, buzzers, air valves, etc. Mutual friend later told me he found the problem, but couldn't remember what it was.

 

Gremlins.