CDL knowledge tests
The air brake test: the numbers and checks you have to know
The air brake section is the most number-heavy test in the CDL stack, and the numbers turn out to be the easy part once you know why each one exists.
An air brake system does not push the brakes on with air. It holds them off with air, and the springs do the braking. Once that is the right way round in your head, most of this test stops being memorisation.
Every number below comes from the AAMVA model CDL manual, which nearly every state adopts. Check them against your own state's manual before test day — that is the document your examiner works from.
The three systems
An air brake setup is really three systems sharing one supply of compressed air:
- Service brakes — what happens when you press the pedal.
- Parking brakes — the spring brakes, held off by air, applied by the springs.
- Emergency brakes — the same springs, applied automatically when air pressure is lost.
That third one is the safety design: lose your air and the truck stops itself. It is also why you never drive with a warning light on, and never cage a spring brake just to move a truck.
The numbers
| What | Value | Why it matters |
|---|---|---|
| Governor cut-out | about 125 psi | Compressor stops pumping |
| Governor cut-in | about 100 psi | Compressor starts again |
| Safety valve setting | 150 psi | Last defence on the supply tank |
| Low-pressure warning | before 60 psi | Light and buzzer must come on |
| Spring brakes apply | between 20 and 45 psi | The truck stops itself |
| Minimum before moving | 100 psi | Build to governor cut-out first |
The leakage test
Engine off, wheels chocked. Maximum air loss per minute:
- Brakes released: 2 psi for a single vehicle, 3 psi for a combination.
- Brakes applied: 3 psi for a single vehicle, 4 psi for a combination.
The pattern is easy to hold onto: applying the brakes allows one more psi, and adding a trailer allows one more psi. Two, three, three, four.
The check, in order
- Engine off, wheels chocked, parking brakes released.
- Time the air loss with the brakes released, then with the pedal held down.
- Start the engine and fan the brakes down: the warning light and buzzer must come on before 60 psi.
- Keep fanning: the spring brakes must pop out between 20 and 45 psi.
- Start the engine again and build back to governor cut-out.
- Test the parking brake: set it, put the vehicle in low gear, pull gently against it. It should hold.
- Test the service brake: roll at about 5 mph and press the pedal firmly. Watch for pulling to one side, delay, or an unusual feel.
What the test is trying to catch
Brake fade versus brake failure. Fade comes from heat — from riding the brakes down a long grade. The cure is the right gear before the descent, not more pedal on the way down.
Wet tanks. Drain them daily. Water condenses inside, and in winter it freezes exactly where you least want ice.
Stab braking versus pumping. In an emergency without ABS: brake hard, release when the wheels lock, reapply when they roll again. Never hold locked wheels — a sliding tyre steers nowhere.
Stopping distance. With air brakes you add brake lag, roughly half a second for the air to travel, on top of perception and reaction distance. That is why total stopping distance at highway speed runs to the length of a football field, and why the honest answer on the test is always the longer one.
Learn the numbers, then take the questions in the real three-option format until what you are using is the reasoning rather than the recall.