CDL Air Brakes Cheat Sheet 2026: 7 Numbers You Must Memorize

Study Guide 4 min read

If you fail the Air Brakes test, you will be restricted to driving vehicles without air brakes (Restriction L). This makes your Class A CDL almost useless.

The hardest part of this endorsement is memorizing the specific PSI (Pounds per Square Inch) numbers and the sequence of the leak test. We’ve compiled them into this simple cheat sheet.

The 7 "Magic Numbers" of Air Brakes

Memorize this table. 50% of your exam questions will come from here.

1. Compressor Cut-Out: 125 PSI
2. Compressor Cut-In: 100 PSI
3. Low Air Warning: 60 PSI
4. Spring Brakes Pop Out: 20-45 PSI
5. Build Rate (85-100 psi): 45 Seconds
6. Max Leak (Static): 3 PSI/min (Combination)
7. Max Leak (Applied): 4 PSI/min (Combination)

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Start Air Brakes Practice

The 3-Step Air Loss Test (LAB)

During your pre-trip inspection, you must perform the brake check in a specific order. Use the acronym L.A.B. to remember it.

L - Leaks

Turn the engine off, turn the key to "on". Push in the brake valves. Press and hold the foot brake. Watch the gauge for 1 minute. You should not lose more than 4 PSI (for Class A).

A - Alarm

Fan the brakes (pump the pedal) to lower the air pressure. The Low Air Warning Light/Buzzer must activate before the pressure drops below 60 PSI.

B - Button (Pop-Out)

Continue fanning the brakes. The Tractor Protection Valve (Yellow Button) and Trailer Supply Valve (Red Button) should pop out automatically when pressure drops between 20 and 45 PSI.

Dual Air Systems Explained

Most modern heavy-duty vehicles use dual air brake systems for safety. This means there are two separate systems sharing a single set of controls.

Primary System: Usually operates the rear axle brakes.
Secondary System: Usually operates the front axle brakes.

You must wait for air pressure to build in BOTH systems (minimum 100 PSI) before driving.

Still confused?

Our App explains every wrong answer so you understand the "Why" behind the rules.

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