Standards for testing and licensing Commercial Motor Vehicle (CMV) drivers are developed and issued by The Federal Highway Administration (FHWA). These standards mean that states can only issue Commercial Driver’s Licenses (CDLs) to CMV drivers if the drivers pass knowledge and skills tests that relate to the type of vehicle the driver will be operating. CMV drivers have been required to have CDLs since April 1, 1992.

Drivers may have a commercial drivers license (CDL) with certain endorsements or restrictions. Endorsements mean that a driver is qualified to operate a particular type of vehicle. Restrictions mean that a driver is not qualified to operate a particular type of vehicle. If a driver fails the air brake component of the general knowledge test or performs the skills test in a vehicle that does not have air brakes, the driver’s CDL will indicate that the driver is restricted from operating a CMV that has air brakes.

How Do Air Brakes Work?

Air brakes use compressed air to make the brakes stop the vehicle. There are actually three braking systems in the air brake system: service brake, parking brake, and emergency brake. The service brake system applies and releases the brakes when the brake pedal is used during normal driving. The parking brake system applies and releases the parking brakes when the parking brake control is used. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.

Stopping distance is greater on vehicles with air brakes because with air brakes there is an added delay when stopping. The delay comes from the time required for the brakes to work after the brake pedal is pushed; it takes one-half second or more for the air to flow through the lines to the brakes. Drivers of CMVs with air brakes need to be aware of brake lag distance so they can allow for additional stopping distance.

What Do CDL Drivers Need to Know about Air Brakes?

All commercial motor vehicle operators must have knowledge of air brakes, unless they have a restriction on the CDL indicating that they are not qualified to operate a CMV with air brakes or have your CDL Air Brakes endorsement. The driver must have knowledge of:

• Air brake system nomenclature


• The risks of a contaminated air supply


• The consequences of severed or disconnected air lines between the power unit and the trailer(s)


• The implications of low air pressure readings


• A method of conducting a safe and accurate pre-trip inspection


• Procedures for performing en-route and post-trip inspections of air actuated brake systems, including the ability to identify defects which may cause the system to fail

Labels: air brakes, cdl, CDL endorsements, commercial drivers license