Low-Pressure Warning Devices
How the system tells you that braking authority is about to be gone — and what to do.
Endorsement: Air Brakes · Source: FMCSA CDL Manual (public domain)
Every air-braked commercial vehicle is required to have a low-pressure warning device that activates before air pressure drops below 60 psi. The warning is typically a combination of a red dashboard light and an audible buzzer; some vehicles also include a wig-wag, a mechanical arm that drops into the driver\'s field of view at the same threshold. The exam may test any of these forms.
The trigger threshold is critical and frequently tested: the device must activate before pressure drops below 60 psi. Some vehicles activate as high as 80 psi. Below 60 psi you have already lost meaningful braking authority on at least one circuit, and the spring brakes will begin to apply somewhere between 20 and 45 psi (the exact range varies by manufacturer but the federal floor is 20 psi). The buzzer is your last warning before automatic emergency-brake application — you should be off the road before the spring brakes apply, because their application is sudden and complete and will not allow you to maneuver.
The correct response to a low-pressure warning is to stop as soon as it is safe to do so. Do not try to continue to your destination. Do not try to pump the brake pedal to recharge the system; pumping a leaking system actually drains air faster from the leak. Pull off the road, set the parking brake (which mechanically applies spring brakes regardless of remaining air), and call for service. The CDL Manual explicitly warns that drivers who ignore low-pressure warnings cause some of the most spectacular runaway-truck crashes investigators see, especially when the warning triggers on a downhill grade.
Key terms to memorize
- compressor
- governor
- supply tank
- service tank
- brake chamber
- slack adjuster
- low-pressure warning
- spring brake
- application gauge
Other Air Brakes topics
- Air Brake System Components — How an air brake system actually moves a brake shoe — every part, in order.
- Seven-Step Pre-Trip Air Brake Test — The mandatory pre-trip sequence that proves your brakes will work when you need them.
- Dual Air Systems — Why modern trucks have two independent air brake systems and how they protect you.
- Parking Brake System — Spring brakes, holding power, and why you never use parking brakes as service brakes.
Test what you learned
Now that you have the Low-Pressure Warning Devices material in your head, drill the Air Brakes practice test. The questions are drawn from the same FMCSA source material this article paraphrases. For state-specific framing, jump to your state page and pick the Air Brakes test for your jurisdiction.