The CDL Air Brakes test is one of the most technically challenging CDL knowledge exams — and one of the most important. If you skip it or fail it, your license will include an "L restriction" that prohibits you from driving any vehicle equipped with air brakes.
What Is the L Restriction?
The L restriction (also called the Air Brakes restriction) is added to your CDL when:
- You fail the Air Brakes knowledge test
- You take your skills test in a vehicle that does NOT have air brakes
With an L restriction, you cannot legally drive tractor-trailers, most heavy dump trucks, transit buses, or any other commercial vehicle equipped with air brakes. Since the majority of Class A and heavy Class B vehicles use air brakes, this restriction severely limits your driving options.
Test Format
- Questions: 25 multiple-choice questions (varies by state)
- Passing score: 80% (20 out of 25)
Key Topics on the Air Brakes Test
Air Brake System Components
Know the function of each major component: the air compressor, air tanks (reservoirs), safety valve, drain valves, brake chambers, slack adjusters, brake shoes/pads, S-cam, and wedge brakes.
Dual Air Brake Systems
Most modern trucks use a dual air brake system — a primary system for rear axles and a secondary system for the front axle. Understand how each system operates independently for safety.
Warning Systems
The low air pressure warning activates at 60 psi. You must know what to do when the warning activates: pull off safely, do not drive until the system is repaired.
Parking Brakes
Air-actuated parking brakes (spring brakes) engage automatically when air pressure drops below a certain level. Know the pop-out warning pressure and spring brake engagement pressures.
Testing Air Brakes
Know the proper procedures for:
- Static pressure test (pressure should not drop more than 3 psi in 1 minute for single vehicles, 4 psi for combination vehicles)
- Low pressure warning test
- Air loss rate test
- Spring brake test
Brake Fade and Overheating
Understand what brake fade is, why it occurs on long downhill grades, and how to prevent it using proper technique (selecting the right gear, using controlled braking).
Study Tips for Air Brakes
- Draw a diagram of the air brake system and label each component
- Memorize the key pressure values: 60 psi warning, 20–45 psi spring brake engagement, 125+ psi maximum tank pressure
- Practice identifying what's being described in questions — the test often asks about unusual situations or failure scenarios
- Take several practice tests specifically focused on Air Brakes before your exam