D.C. Mun. Regs. r. 18-723
Each 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 (50%) 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 (lbs/in2), the pressure available for braking.
Each truck tractor and truck used for towing a vehicle equipped with vacuum operated brakes and every truck with three or more axles using vacuum in the operation of its brakes, except those in driveway or towaway operations, shall be equipped with a warning signal, other than a gauge indicating vacuum, readily audible or visible to the driver, which will operate at any time the vacuum in the vehicle's supply reservoir or reserve capacity is less than eight inches (8 in.) of mercury.
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, but are not required to be, combined into a single device which will serve both purposes. A gauge or gauges indicating pressure or vacuum shall not be deemed to be an adequate means of satisfying this requirement.
D.C. Mun. Regs. r. 18-723