Warning Devices, Low Air Pressure Warning, and Emergency Braking
Warning devices are your last clear notice before the spring brakes take over. The exam tests the pressures and what you are supposed to do.
The warning
An audible buzzer and a red light (often a wig-wag) must come on before reservoir pressure drops below 60 psi. The requirement is that it warns before 60, not that it waits until 20. If it comes on while driving, you have a serious leak: move to the shoulder, stop, and do not continue.
Spring brakes and emergency braking
Spring brakes are held off by air. When pressure falls to roughly 20–45 psi they apply on their own, so a total air loss still stops the vehicle - abruptly.
Modulated emergency braking: pulling the yellow parking knob partway applies the spring brakes gradually instead of slamming them on. Pull it all the way for a full emergency application. The tractor protection valve also closes when the trailer supply line loses pressure or separates, applying the trailer brakes.
Worked example
The buzzer sounds at 58 psi on the interstate with traffic behind. Reasoning: you are minutes from spring brake application. Signal, keep steering, brake smoothly, and get off the road; do not try to reach the next exit.
Traps
Assuming the brakes are fine because the pedal still works; thinking spring brakes wait until 0 psi; using the trailer hand valve as an emergency brake on a tractor-trailer (wrong valve, wrong effect).