The CDL air brakes test: what it covers and the numbers to know
Twenty-five questions, most of them numbers. Every pressure, time and distance from Section 5 of the manual, what it means, and why skipping the test costs you.
The air brakes test is 25 questions, you need 20 right, and it is the test drivers most often underestimate. Section 5 of the CDL manual is short compared with general knowledge, but it is dense with numbers, and the test asks for them exactly. It is not an endorsement. It is a test you take so that your licence does not carry the L restriction, which bans you from any vehicle with full air brakes, meaning most trucks and buses.
How an air brake system works, in one paragraph
An engine-driven compressor pumps air into storage tanks. A governor stops the compressor when the tanks reach the cut-out pressure and restarts it at the cut-in pressure. Pressing the brake pedal (the foot valve) sends air to brake chambers at each wheel; a pushrod turns a slack adjuster, which twists an S-cam that forces the brake shoes against the drum. Release the pedal and springs pull everything back. Separate spring brakes hold the parking brake and apply on their own if air pressure drops too far. Modern vehicles have a dual air system with two circuits, so one leak does not take out all the brakes.
The numbers the test asks for
| What | Figure | Why it matters |
|---|---|---|
| Governor cut-out | About 125 psi | Compressor stops pumping; tanks are full |
| Governor cut-in | About 100 psi | Compressor starts again |
| Air pressure build-up | 85 to 100 psi within 45 seconds (dual system) | Checks the compressor can keep up |
| Low air pressure warning | Must come on before pressure drops below 60 psi | Your signal to stop safely while you still can |
| Spring brakes apply | Between 20 and 45 psi | The parking brakes come on by themselves; you may lose steering control if moving |
| Safety relief valve | Opens at about 150 psi | Protects the tanks from over-pressure; if it opens, something is wrong |
| Applied leakage, single vehicle | No more than 3 psi lost in one minute | Engine off, brake pedal held after the initial drop |
| Applied leakage, combination | No more than 4 psi in one minute | Same test with a trailer connected |
| Static leakage, single vehicle | No more than 2 psi in one minute | Engine off, brakes released |
| Static leakage, combination | No more than 3 psi in one minute | |
| Brake lag distance | About 32 feet at 55 mph | Air takes about half a second to reach the brakes |
| Slack adjuster free play | More than about 1 inch means it needs adjustment | Checked by pulling hard on it by hand with the brakes released |
The test does not ask you to memorise the whole table at once; each question picks one figure. But a wrong figure is the single most common reason for a failed air brakes test, so these twelve lines are the core of your study.
The checks you must be able to describe
Section 5 ends with the inspection steps, and the test asks about them as procedures:
- Drain the tanks. Air tanks collect water and compressor oil, which can freeze in winter. Manually drained tanks are drained daily; automatic drains still need checking.
- Check the compressor belt, if it has one, for tightness.
- Low pressure warning test. With the engine running, fan the brake pedal to drop pressure; the warning light or buzzer must come on before 60 psi.
- Spring brake test. Keep fanning; the parking brake knob should pop out between 20 and 45 psi.
- Build-up test. Run the engine at a fast idle; pressure should build from 85 to 100 psi within 45 seconds.
- Leakage tests, applied and static, with the figures above.
- Parking brake test. With the parking brake set, gently pull against it in low gear; the vehicle should not move.
- Service brake test. At about 5 mph, press the brake firmly; the vehicle should stop straight with no pulling or delay.
Stopping distance with air brakes
The manual splits stopping distance into perception distance, reaction distance, brake lag distance and braking distance. Air brakes add the lag: roughly half a second between pressing the pedal and the brakes applying, which at 55 mph is about 32 feet. The practical rule the test wants is that air-braked vehicles need more following distance than cars, and much more on wet roads or downhill.
Downhill and emergency rules
Two more topics appear on nearly every version of the test:
- Going downhill: use a low gear and the engine to control speed, and brake with controlled braking: press hard enough to feel a definite slowdown, let off until about 5 mph above your safe speed, then repeat. Riding the brakes lightly all the way down overheats them and they fade.
- Emergency stops: either controlled braking (hard, without locking the wheels) or stab braking (brake fully, release when the wheels lock, reapply when they roll again). Antilock brakes (ABS) let you brake hard and steer; they do not shorten your stopping distance in normal conditions.