Vehicle Code § 26521

Motor vehicles required to be equipped with power brakes and equipped with vacuum or vacuum-assisted brakes and motor vehicles used to tow vehicles equipped with vacuum brakes or vacuum-assisted brakes shall be equipped with either an audible or visible warning signal to indicate readily to the driver when the vacuum drops to eight inches of mercury and less. A vacuum gauge shall not be deemed to meet this requirement.

This section shall not apply to a two-axle motor truck operated singly nor to any motor vehicle manufactured prior to 1964.