Title 49, § 393.43
Breakaway & Emergency
(a) Every motor vehicle, if used to tow a trailer equipped with
brakes, shall be equipped with means for providing that in case
of breakaway of such trailer the service brakes on the towing vehicle
will be sufficiently operative to stop the towing vehicle.
(b) Every truck or truck tractor equipped with air brakes, when
used for towing other vehicles equipped with air brakes, shall be
equipped with two means of activating the emergency features of
the trailer brakes. One of these means shall operate automatically
in the event of reduction of the towing vehicle air supply to a
fixed pressure which shall not be lower than 20 pounds per square
inch nor higher than 45 pounds per square inch. The other means
shall be a manually controlled device readily operable by a person
seated in the driving seat. Its emergency position or method of
operation shall be clearly indicated. In no instance may the manual
means be so arranged as to permit its use to prevent operation of
the automatic means. The automatic and manual means required by
this section may be, but are not required to be, separate.
(c) Every truck tractor and truck when used for towing other vehicles
equipped with vacuum brakes, shall have, in addition to the single
control required by Sec. 393.49 to operate all brakes of the combination,
a second manual control device which can be used to operate the
brakes on the towed vehicles in emergencies. Such second control
shall be independent of brake air, hydraulic, and other pressure,
and 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 by this rule to provide modulated
or graduated braking.
(d) Every trailer required to be equipped with brakes shall be
equipped with brakes of such character as to be applied automatically
and promptly upon breakaway from the towing vehicle, and means shall
be provided to maintain application of the brakes on the trailer
in such case for at least 15 minutes.
(e) Air brake systems installed on towed vehicles shall be so designed,
by the use of ``no-bleed-back'' relay emergency valves or equivalent
devices, that the supply reservoir used to provide air for brakes
shall be safeguarded against backflow of air to the towing vehicle
upon reduction of the towing vehicle air pressure.
(f) The requirements of paragraphs (b), (c), and (d) of this section
shall not be applicable to motor vehicles in driveaway-towaway operations.