Since 1992, drivers of certain commercial vehicles must prove they have mastered specific skills to obtain a commercial driver's license, or CDL, to legally operate those vehicles. Because many of these large vehicles use air brakes--braking systems powered by compressed air--drivers must become certified in air brakes as part of the knowledge and skills tests required to be issued a CDL. Though some details vary among states, the Commercial Motor Vehicle Safety Act of 1986 establishes minimum federal standards for air-brake certification. A CDL test consists of a written and a hands-on skills test. The skills test must be taken in a vehicle equipped with air brakes to become certified.

Knowledge Test

Demonstrate knowledge of the correct terms for components of an air-brake system.

Demonstrate knowledge of the dangers of a contaminated air-brake system.

Demonstrate knowledge of what will happen if an air line between the air-brake power unit and a towed trailer becomes damaged or severed.

Demonstrate knowledge of the implications of a low air-pressure reading.

Demonstrate knowledge of how to conduct a safety inspection of the air-braking system before, during and after a trip, including how to identify any defects that may cause a system failure.

Skills Test

Locate and identify operating controls and monitoring devices, such as an air-pressure gauge, for the brake system.

Determine the condition of the brake system, including connections to a towed trailer or trailers, and make any necessary adjustments.

Inspect low-pressure warning systems to make sure they work.

Determine that the system will maintain enough air pressure to properly operate the brakes.

Make sure air pressure builds up in an acceptable amount of time and that alarms and other emergency devices shut off at the proper pressure level.

Operate the vehicle to determine if the brake system is working properly.