Generally speaking, operating a vehicle with air brakes requires a commercial driver's license class A, B or C, as all three classes feature an air brakes knowledge test during certification. Inability to pass this test earns the L restriction on a commercial driver's license.
The L restriction indicates that the license holder may not operate a commercial motor vehicle with a full air brake system. Likewise, the Z restriction indicates that the licensee did not pass the hydraulic brake knowledge test. Typically, large vehicles, such as tractor trailers, buses and heavy-duty trucks use air brakes as they are more effective and reliable than hydraulic brakes. Air-brake systems are fail-safe, as the lack of positive air pressure engages the braking system gradually, preventing run-away vehicles, except in cases where the brake pads are overheated and cannot engage correctly.