D.C. Mun. Regs. r. 18-721
Each trailer, semi-trailer, and pole trailer equipped with air or vacuum actuated brakes and every trailer, semi-trailer, and pole trailer with a gross weight in excess of three thousand pounds (3,000 lbs.), manufactured or assembled after September 15, 1963, shall be equipped with brakes acting on all wheels which will be applied automatically and promptly, and remain applied for at least fifteen (15) minutes, upon breakaway from the towing vehicle.
Every motor vehicle manufactured or assembled after September 15, 1963, and used to tow a trailer, semi-trailer or pole trailer equipped with brakes, shall be equipped with means for providing that, in case of breakaway of the towed vehicle, the towing vehicle will be capable of being stopped by the use of its service brakes.
Air brake systems installed on trailers manufactured or assembled after September 15, 1963, shall be so designed that the supply reservoir used to provide air for the brakes shall be safeguarded against backflow of air from the reservoir through the supply line.
Each towing vehicle, when used to tow another vehicle equipped with air controlled brakes in other than driveway or towaway operations, shall be equipped with two (2) means for emergency application of the trailer brakes.
One of the means for emergency application of trailer brakes shall apply the brakes automatically in the event of a reduction of the towing vehicle air supply to a fixed pressure which shall not be lower than twenty pounds per square inch (20 lbs./in.) or higher than forty-five pounds per square inch (45 lbs./in.).
The other means for emergency application of the trailer brakes shall be a manually controlled device for applying and releasing the brakes, readily operable by a person seated in the driving seat and its emergency position or method of operation shall be clearly indicated. In no instance may the manual means be arranged to permit its use to prevent operation of the automatic means. The automatic and manual means required by this section may be separate, but are not required to be separate.
Each towing vehicle used to tow other vehicles equipped with vacuum brakes in operations other than driveway or towaway operations, shall have, in addition to the single control device required by § 720.10, a second control device which can be used to operate the brakes on towed vehicles in emergencies.
The second control required by § 721.7 shall be independent of other controls, unless the braking system be so arranged that failure of the pressure on which the second control depends will cause the towed vehicle brakes to be applied automatically. The second control is not required to provide modulated braking.
D.C. Mun. Regs. r. 18-721