D.C. Mun. Regs. tit. 18, r. 18-722
Each bus, truck or truck tractor with air-operated brakes shall be equipped with at last one (1) reservoir sufficient to ensure that, when fully charged to the maximum pressure as regulated by the air compressor governor cut-out setting a full service brake application may be made without lowering such reservoir pressure by more than twenty per cent (20%). Each reservoir shall be provided with means for readily draining accumulated oil or water.
Each truck with three (3) or more axles equipped with vacuum assistor-type brakes and every truck tractor and truck used for towing a vehicle equipped with vacuum brakes shall be equipped with a reserve capacity or a vacuum reservoir sufficient to ensure that, with the reserve capacity or reservoir fully charged and with the engine stopped, a full service brake application may be made without depleting the vacuum supply by more than forty percent (40%).
All motor vehicles, trailers, semi-trailers and pole trailers, when equipped with air or vacuum reservoirs or reserve capacity as required by this chapter, shall have the reservoirs or reserve capacity safeguarded by a check valve or equivalent device so that, in the event of failure or leakage in its connection to the source of compressed air or vacuum, the stored air or vacuum shall not be depleted by the leak or failure.
D.C. Mun. Regs. tit. 18, r. 18-722