CDL air brakes test: what it covers and how to pass
The air brakes test is a knowledge test you take for the CDL permit if you will drive a vehicle with air brakes, which is almost every Class A truck. Most states use the national model: typically 25 questions, 20 correct to pass. Skip it or fail it and your license carries an air brake restriction. Most of the test comes down to a handful of pressure numbers and how the system works.
- Typically 25 questions; 20 right (80%) to pass. Check your state manual for the exact count.
- Covers section 5 of the CDL manual: the parts of the system, dual systems, inspecting it, and using it.
- A few numbers differ by state manual, most famously the low air warning: 55 psi in the national model, 60 in some states’ air brakes chapter.
The numbers you must know
These come up again and again, worded different ways. Know them cold.
| What | The number |
|---|---|
| Governor cut-out | Around 125 psi: the compressor stops pumping |
| Governor cut-in | Around 100 psi: the compressor starts again |
| Safety relief valve | Usually opens at 150 psi |
| Low air warning | Before pressure drops below 55 psi in the national model manual. Several state manuals still say 60 psi in the air brakes chapter (for example New York, North Carolina and Washington): answer with your state manual’s figure |
| Spring brakes come on | Between 20 and 45 psi, typically 20 to 30 |
| Static leakage (brakes released) | Less than 2 psi a minute on a single vehicle, 3 on a combination |
| Brake lag at 55 mph | About 32 extra feet of stopping distance |
| Total stopping distance at 55 mph | Over 450 feet for an air-braked vehicle |
Don’t mix up the two leakage checks. The knowledge test asks about the static check (brakes released): under 2 psi a minute single, 3 combination. The skills test’s air brake check uses the applied check (brake held): no more than 3 single, 4 combination. How to do the air brake check.
What the test covers
- The parts of the system: compressor and governor, air tanks and drains, safety relief valve, brake pedal, foundation brakes (drums, shoes, linings, S-cams), slack adjusters, gauges, the low air warning, spring brakes, ABS.
- Dual air systems: two separate systems, a gauge for each half or one gauge with two needles.
- Inspecting the system: checking slack adjusters with the vehicle on level ground and chocked, testing the governor, leakage, the low air warning and spring brakes.
- Using air brakes: brake lag and stopping distance, controlled and stab braking, brake fade on long downgrades, why you drain the tanks (water can freeze and cause brake failure).
How to pass it
- Read section 5 of your state’s CDL manual once, start to finish. It is short.
- Learn the numbers in the table above, then check them against your state’s manual: where they differ, the test follows your manual.
- Take practice tests until you score 90% or more every time, not just once.
- For every question you miss, read why the right answer is right. That is where the numbers stick.
Start with the free air brakes practice test, or pick your state on the practice tests page for the version written from your manual.
Every air brakes question, explained
Practice the full air brakes bank in the app. Every answer says why it's right and which handbook section it comes from, and the AI coach explains any miss in plain words. Questions follow your state's numbers.
Practice air brakes in the appRelated guides
Questions people ask
Most states use the national model: typically 25 questions with 20 correct needed to pass (80%). Your state manual has the exact count.
Your CDL gets an air brake restriction, so you cannot drive a vehicle with air brakes. Almost every Class A truck has them.
Before the pressure drops below 55 psi in the national model manual. Several state manuals still say 60 psi in the air brakes chapter (for example New York, North Carolina and Washington). On the written test, answer with the figure in your own state’s manual.
Between 20 and 45 psi, typically 20 to 30 psi.