CDL Air Brakes Endorsement

Class A & B Updated for 2026

Preparing for the 2026 CDL Air Brakes Practice Test? You are in the right place. The Air Brakes endorsement is mandatory for most Class A (Semi-trucks) and Class B (Buses/Dump Trucks) vehicles. If you don't understand how the air pressure system works—specifically the L.A.B. check—you will fail instantly.

The "Cheat Sheet" Numbers

Memorize These

About 50% of the questions on the actual CDL Air Brakes Practice Test (and the real DMV exam) rely on knowing these specific PSI thresholds.

Component / Event	PSI Reading
Compressor Cut-Out Pressure	125 psi
Compressor Cut-In Pressure	100 psi
Low Air Warning Alarm	Must on by 60 psi
Spring Brakes Activate (Pop-out)	20 - 45 psi
Safety Relief Valve Opens	150 psi

Why the Air Brakes Test is Hard

Unlike the General Knowledge test, which uses a lot of common sense, the air brakes section is highly technical. Taking a reliable CDL Air Brakes Practice Test is crucial because you need to understand the physics of air pressure, how the parking brake interacts with the service brake, and what "brake lag" means for your stopping distance.

The DMV exam will ask you tricky questions about the "S-Cam," the "Slack Adjuster," and the "Air Compressor Governor." Our guide below breaks these down so you can ace your CDL Air Brakes Practice Test on the first try.

Technical Concepts Decoded

The L.A.B. Check (Critical)

This is the most common reason for failure. Remember the acronym L.A.B. for the 3-step air loss test:

L - Leaks: Turn engine off, turn key on. Release parking brake. Fully apply foot brake. Hold for 1 minute. Max loss: 3 psi (single) / 4 psi (combination).
A - Alarm: Fan the brakes until the Low Air Warning light/buzzer comes on. Must activate by 60 psi.
B - Button: Continue fanning brakes. The parking brake valve (yellow button) should pop out. Between 20-45 psi.

Dual Air Systems

Modern trucks have two separate air systems (Primary and Secondary). If one fails, the other can still stop the truck, but you must know how to read the dual gauges.

S-Cam Brakes & Slack Adjusters

The most common type of foundation brake is the S-Cam. You must know that the S-Cam forces the brake shoes against the inside of the brake drum. Also, CDL Air Brakes Practice Test questions often ask about "Slack Adjusters"—these adjust the wear on the brakes and shouldn't move more than 1 inch when pulled by hand.