Title 49, § 393.50
(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