61-9-311. Warning devices. (1) A bus, truck, or truck tractor using compressed air for the operation of its own brakes or the brakes on a towed vehicle must have a warning signal, other than a pressure gauge, readily audible or visible to the driver that will operate at any time the air reservoir pressure of the vehicle is below 60 pounds per square inch. In addition, each vehicle must be equipped with a pressure gauge visible to the driver that indicates in pounds per square inch the pressure available for braking.

(2) A truck tractor, truck used for towing a vehicle equipped with vacuum operated brakes, or a truck with three or more axles using vacuum in the operation of its brakes, except those in driveaway or towaway operations, must be equipped with a warning signal, other than a gauge indicating vacuum, readily audible or visible to the driver that will operate at any time the vacuum in the vehicle's supply reservoir or reserve capacity is less than 8 inches of mercury.

(3) When a vehicle required to be equipped with a warning device is equipped with both air and vacuum power for the operation of its own brakes or the brakes on a towed vehicle, the warning devices may be combined into a single device that will serve both purposes. A gauge or gauges indicating pressure or vacuum is not an adequate means of satisfying this requirement.