The Air Brakes (L) endorsement is mandatory for most Class A and Class B vehicles. Unlike the General Knowledge exam, this test requires memorizing specific PSI numbers. Our AI-powered cdl practice test air brakes simulator covers every technical detail you need to know.
It's not about driving skill; it's about physics and systems. If you fail to identify a leak during the L.A.B. Check, you fail the entire exam immediately.
At what PSI should the Low Air Warning alarm activate?
What is the maximum leakage rate for a combination vehicle (truck + trailer)?
About 50% of the questions on the cdl practice test air brakes (and the real DMV exam) rely on knowing these specific PSI thresholds. Memorize this table before you walk into the DMV.
| Component / Event | PSI Reading |
|---|---|
| Compressor Cut-Out | 125 psi |
| Compressor Cut-In | 100 psi |
| Low Air Warning Alarm | Must be on by 60 psi |
| Spring Brakes Activate (Pop-out) | 20 - 45 psi |
| Safety Relief Valve Opens | 150 psi |
You must perform this check in the correct order. Use the acronym L.A.B. to remember it.
Turn engine OFF, key ON. Release parking brake. Fully apply foot
brake. Hold for 1 minute.
Max Loss: 3 psi (single vehicle) / 4
psi (combination).
Fan the brake pedal to lower pressure. The warning light/buzzer MUST activate before pressure drops below 60 psi.
Continue fanning. The yellow parking brake button MUST pop out between 20 and 45 psi.
"I failed twice because I mixed up the cut-out and cut-in pressures. This cheat sheet saved me."
"The L.A.B. check explanation is better than what my instructor taught me. Highly recommend."
Don't let an "L" restriction hold back your career. Use our cdl practice test air brakes to memorize the PSI numbers today.
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