[DK on the road]

All commercial vehicles with Air brakes built since about the late 50's', that I have been in, have had a buzzer that goes on at any drop below 75 psi. I believe its a federal safety law, not state. By the way, that question is on the CDL tests, at least in CA. Not sure how a Newell doesn't come them.

#784 has a chime that goes off for the step, not the air. I'm not in any hurry to find and activate it.

I had a 97' Air brake coach and I added tape over the buzzer. It made noise after the tape, just not to the annoying level it was designed for. I always had to air up that coach, at startup, and it would drive the dog and kids crazy for a few minutes.