California Code, Vehicle Code - VEH § 15260

(a) Any applicant for a commercial driver's license who does not successfully complete the air-brake component of the knowledge test, or who does not successfully complete the driving-skill test in a vehicle or combination of vehicles equipped with air brakes, shall, if otherwise qualified, receive a commercial driver's license that restricts the licenseholder from operating a commercial motor vehicle equipped with air brakes.

(b) To remove the restriction described in subdivision (a) from a commercial driver's license, the driver is required to make a new application for a commercial driver's license, and, in addition to any other requirements specified in this code, to successfully complete the air-brake component of the knowledge test prescribed by the department, and to pass the driver-skill test in a vehicle or combination of vehicles equipped with air brakes.

(c) For the purposes of the driving-skill test and the restriction described in this section, air brakes shall include any braking system operating fully or partially on the air-brake principle.