Low Air Warnings & Spring Brakes
10 free questions, part of the Air Brakes CDL practice test
The low air pressure warning light and buzzer must activate before pressure in either circuit of a dual air brake system drops below 55 psi. If that warning comes on while you're driving, pull off the road and stop safely right away, because spring brakes apply automatically once air pressure drops too low to hold them off. ELDT.io's free practice bank has 10 questions on low air warnings and spring brakes.
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What triggers the low air pressure warning light on a CDL vehicle?
The low air pressure warning light and buzzer turn on when pressure in either the primary or secondary air circuit drops below 55 psi. On large buses, the warning devices are often set to activate at a higher pressure than that.
What should you do if the low air warning comes on while driving?
Stop as soon as you safely can. Don't keep driving on a falling air supply, because once pressure drops low enough the spring brakes apply on their own, and you don't want that happening while you're still moving.
Why do commercial vehicles have spring brakes?
Spring brakes are a mechanical fail-safe. They're held off by air pressure, so if the vehicle loses air pressure entirely, powerful springs push the brakes on automatically instead of leaving the vehicle with no brakes at all.
What's the difference between the primary and secondary air brake circuits?
A dual air brake system splits braking power across two separate circuits so a failure in one doesn't leave you with no brakes at all. Before driving, you should let the compressor build at least the minimum required pressure in both systems, not just one.