2005 California Vehicle Code Sections 26520-26522 Article 4. Vacuum Brakes

26520.  Motor vehicles required to be equipped with power brakes and
which are equipped with vacuum or vacuum-assisted brakes shall be
equipped with a properly maintained vacuum gauge of reliable and
satisfactory construction, accurate within 10 percent of the actual
vacuum in the supply reservoir, and visible and legible to the driver
at all times.
   This section shall not apply to a two-axle motor truck operated
singly.
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.
26522.  Vehicles required to be equipped with power brakes and
equipped with vacuum or vacuum-assisted brakes shall have a check
valve installed and properly maintained in the vacuum system between
the source of vacuum and the vacuum reserve.