In California, an air brake endorsement is required to operate a commercial motor vehicle (CMV) with air brakes, regardless of the vehicle's weight. CMVs are defined as vehicles with a gross vehicle weight rating (GVWR) over 26,000 pounds, or vehicles designed to transport 16 or more passengers (including the driver).
So if you plan to operate a vehicle weighing under 26,000 pounds equipped with air brakes, you need an air brake endorsement on your driver's license.