SDCL § 32-18-20: Buses and trucks with air brakes--Warning signal for low air pressure--Pressure gauge--Violation as misdemeanor.

Every bus, truck, or truck tractor using compressed air for the operation of its own brakes or the brakes on any towed vehicle, shall be provided with a warning signal, other than a pressure gauge, readily audible or visible to the driver, which will operate at any time the air reservoir pressure of the vehicle is below fifty percent of the air compressor governor cut-out pressure. In addition, each such vehicle shall be equipped with a pressure gauge visible to the driver, which indicates in pounds per square inch the pressure available for braking. A violation of this section is a Class 2 misdemeanor.