• Title 49 Part 571.121 -- Air Brake Systems
• Title 49 Part 393.40 -- Required Systems
• Title 49 Part 393.43 -- Breakaway & Emergency
• Title 49 Part 393.45-46 -- Brake Tubing
• Title 49 Part 393.50 -- Reservoirs
• Title 49 Part 393.55 -- Anti-Lock Brakes

• Title 49 Part 571 Section 106

• Title 49 Part 571.108 -- Lamps, Reflective Devices, And Associated Equipment
• Title 49 Part 393.11 -- Lights & Reflectors
• Title 49 Part 393.19 -- Turn Signals

(a) General. Every commercial motor vehicle using air or vacuum for breaking shall be equipped with reserve capacity or a reservoir sufficient to ensure a full service brake application with the engine stopped without depleting the air pressure or vacuum below 70 percent of that pressure or degree of vacuum indicated by the gauge immediately before the brake application is made. For purposes of this section, a full service brake application is considered to be made when the service brake pedal is pushed to the limit of its travel.

(b) Safeguarding of air and vacuum.

(1) Every bus, truck, and truck tractor, when equipped with air or vacuum reservoirs and regardless of date of manufacture, shall have such reservoirs so safeguarded by a check valve or equivalent device that in the event of failure or leakage in its connection to the source of compressed air or vacuum the air or vacuum supply in the reservoir shall not be depleted by the leak or failure.

(2) Means shall be provided to establish the check valve to be in working order. On and after May 1, 1966, means other than loosening or disconnection of any connection between the source of compressed air or vacuum and the check valve, and necessary tools for operation of such means, shall be provided to prove that the check valve is in working order. The means shall be readily accessible either from the front, side, or rear of the vehicle, or from the driver's compartment.

(i) In air brake systems with one reservoir, the means shall be a cock, valve, plug, or equivalent device arranged to vent a cavity having free communication with the connection between the check valve and the source of compressed air or vacuum.

(ii) Where air is delivered by a compressor into one tank or compartment (wet tank), and air for braking is taken directly from another tank or compartment (dry tank) only, with the required check valve between the tanks or compartments, a manually operated drain cock on the first (wet) tank or compartment will serve as a means herein required if it conforms to the requirements herein.

(iii) In vacuum systems stopping the engine will serve as the required means, the system remaining evacuated as indicated by the vacuum gauge.