Question 1
After the governor has stopped the compressor, tank pressure slowly falls as you use the brakes. At about what pressure does the governor let the compressor start pumping again?
The CDL air brakes test
If you will drive a vehicle with air brakes, you take the air brakes knowledge test. It covers section 5 of the CDL manual, and you need 80% to pass. Fail it and your license carries an air brake restriction.
| Who takes it | Drivers of vehicles with air brakes, including air over hydraulic (California handbook) |
|---|---|
| Pass mark | Each test: at least 80% right (49 CFR 383.135) |
| Below 80% | An air brake restriction on the CLP and CDL |
| Manual section | 5: parts, dual systems, inspecting, using |
What it covers
- 5.1 The parts of an air brake system: compressor, governor, tanks, drains, safety valve, brake chambers, slack adjusters, gauges, warnings and spring brakes.
- 5.2 Dual air brake systems: two systems, two gauges, and building to at least 100 psi in both before driving.
- 5.3 Inspecting air brakes: the extra checks in the seven-step inspection, including leakage, the low air warning and the spring brakes.
- 5.4 Using air brakes: normal and emergency stops, brake lag, stopping distance and braking on long downgrades.
The numbers to know
- The governor stops the compressor at about 125 psi and starts it again at about 100 psi.
- The safety valve is usually set to open at 150 psi.
- The low air warning comes on before tank pressure falls below 55 psi.
- Spring brakes come on between 20 and 45 psi in most vehicles.
- Brake lag adds about 32 feet at 55 mph, so total stopping distance is over 450 feet.
Try 3 questions
Question 2
Air is slowly leaking from a straight truck's brake system. In what pressure range will the spring brakes come fully on?
Question 3
Wearing gloves, you pull hard on a slack adjuster and it moves about one and a half inches where the push rod attaches. What does this tell you?
Next step
Questions people ask
What score do I need on the CDL air brakes test?
At least 80% right, as on every CDL knowledge test.
What happens if I fail the air brakes test?
If you are issued a permit or license anyway, it carries an air brake restriction, so you may not drive a vehicle with air brakes.
Is the air brakes test the same in every state?
States build it from the same manual section and a question pool approved by FMCSA, so the content is the same everywhere.