[Code of Federal Regulations]
[Title 49, Volume 5]
[Revised as of October 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR393.43]

[Page 402-403]
 
                        TITLE 49--TRANSPORTATION
 
                      DEPARTMENT OF TRANSPORTATION
 
PART 393_PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATION--Table of 
Contents
 
                            Subpart C_Brakes
 
Sec.  393.43  Breakaway and emergency braking.

    (a) Towing vehicle protection system. Every motor vehicle, if used 
to tow a trailer equipped with brakes, shall be equipped with a means 
for providing that in the case of a breakaway of the trailer, the 
service brakes on the towing vehicle will be capable of stopping the 
towing vehicle. For air braked towing units, the tractor protection 
valve or similar device shall operate automatically when the air 
pressure on the towing vehicle is between 138 kPa and 310 kPa (20 psi 
and 45 psi).
    (b) Emergency brake requirements, air brakes. Every truck or truck 
tractor equipped with air brakes, when used for towing other vehicles 
equipped with air brakes, shall be equipped with two

[[Page 403]]

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) Emergency brake requirements, vacuum brakes. 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) Breakaway braking requirements for trailers. Every trailer 
required to be equipped with brakes shall have brakes which apply 
automatically and immediately upon breakaway from the towing vehicle. 
With the exception of trailers having three or more axles, all brakes 
with which the trailer is required to be equipped must be applied upon 
breakaway from the towing vehicle. The brakes must remain in the applied 
position for at least 15 minutes.
    (e) Emergency valves. 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) Exception. The requirements of paragraphs (b), (c) and (d) of 
this section shall not be applicable to commercial motor vehicles being 
transported in driveaway-towaway operations.

[53 FR 49384, Dec. 7, 1988, as amended at 70 FR 48050, Aug. 15, 2005]