Section 26505 Of Article 3. Airbrakes From California Vehicle Code >> Division 12. >> Chapter 3. >> Article 3.

. A motor vehicle equipped with airbrakes or equipped to operate airbrakes on towed vehicles shall be equipped with a pressure gauge of reliable and satisfactory construction and maintained in an efficient working condition, accurate within 10 percent of the actual air reservoir pressure, and visible and legible to a person when seated in the driving position.