California Code, Vehicle Code - VEH § 26506

(a) Every motor vehicle airbrake system used to operate the brakes on a motor vehicle or on a towed vehicle shall be equipped with a low air pressure warning device that complies with either the requirements set forth in the Federal Motor Vehicle Safety Standards in effect at the time of manufacture or the requirements of subdivision (b).

(b) The device shall be readily visible or audible to the driver and shall give a satisfactory continuous warning when the air supply pressure drops below a fixed pressure, which shall be not more than 75 pounds per square inch nor less than 55 pounds per square inch with the engine running. A gauge indicating pressure shall not satisfy this requirement.