Texas Transportation Code - TRANSP § 547.407. Brake Warning Devices Required

(a) A bus, truck, or truck-tractor that uses air to operate its brakes or the brakes of a towed vehicle shall be equipped with:

(b) A truck-tractor or truck used to tow a vehicle equipped with vacuum brakes, or a truck with at least three axles that is equipped with vacuum brakes, shall be equipped with a warning signal, other than a gauge showing vacuum, that is readily audible or visible to the vehicle operator and that shows when the vacuum in the reservoir or reserve capacity is less than eight inches of mercury. This subsection does not apply to an operation in which a motor vehicle, trailer, or semitrailer is transported as a commodity when at least one set of the vehicle's wheels is on the roadway.

(c) If a vehicle required to be equipped with a warning device is equipped with air and vacuum power to operate its brakes or the brakes on a towed vehicle, the warning devices required may be combined into a single device that is not a pressure or vacuum gauge.