[Code of Federal Regulations]
[Title 49, Volume 4]
[Revised as of October 1, 2003]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR393.51]

[Page 1110]
 
                        TITLE 49--TRANSPORTATION
 
        CHAPTER III--FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION,
                      DEPARTMENT OF TRANSPORTATION
 
PART 393--PARTS AND ACCESSORIES NECESSARY FOR SAFE OPERATION--Table of Contents
 
                            Subpart C--Brakes
 
Sec. 393.51  Warning devices and gauges.

    (a) General. In the manner and to the extent specified in paragraphs 
(b), (c), (d), and (e) of this section, a bus, truck, or truck tractor 
must be equipped with a signal that provides a warning to the driver 
when a failure occurs in the vehicle's service brake system.
    (b) Hydraulic brakes. A vehicle manufactured on or after July 1, 
1973, and having service brakes activated by hydraulic fluid must be 
equipped with a warning signal that performs as follows:
    (1) If Federal Motor Vehicle Safety Standard No. 105 (Sec. 571.105 
of this title) was applicable to the vehicle at the time it was 
manufactured, the warning signal must conform to the requirements of 
that standard.
    (2) If Federal Motor Vehicle Safety Standard No. 105 (Sec. 571.105) 
was not applicable to the vehicle at the time it was manufactured, the 
warning signal must become operative, before or upon application of the 
brakes in the event of a hydraulic-type complete failure of a partial 
system. The signal must be readily audible or visible to the driver.
    (c) Air brakes. A vehicle (regardless of the date it was 
manufactured) having service brakes activated by compressed air (air-
mechanical brakes) or a vehicle towing a vehicle having service brakes 
activated by compressed air (air-mechanical brakes) must be equipped, 
and perform, as follows:
    (1) The vehicle must have a low air pressure warning device that 
conforms to the requirements of either paragraph (c)(1) (i) or (ii) of 
this section.
    (i) If Federal Motor Vehicle Safety Standard No. 121 (Sec. 571.121 
of this title) was applicable to the vehicle at the time it was 
manufactured, the warning device must conform to the requirements of 
that standard.
    (ii) If Federal Motor Vehicle Safety Standard No. 121 (Sec. 571.121) 
was not applicable to the vehicle at the time it was manufactured, the 
vehicle must have a device that provides a readily audible or visible 
continuous warning to the driver whenever the pressure of the compressed 
air in the braking system is below a specified pressure, which must be 
at least one-half of the compressor governor cutout pressure.
    (2) The vehicle must have a pressure gauge which indicates to the 
driver the pressure in pounds per square inch available for braking.
    (d) Vacuum brakes. A vehicle (regardless of the date it was 
manufactured) having service brakes activated by vacuum or a vehicle 
towing a vehicle having service brakes activated by vacuum must be 
equipped with--
    (1) A device that provides a readily audible or visible continuous 
warning to the driver whenever the vacuum in the vehicle's supply 
reservoir is less than 8 inches of mercury; and
    (2) A vacuum gauge which indicates to the driver the vacuum in 
inches of mercury available for braking.
    (e) Hydraulic brakes applied or assisted by air or vacuum. A vehicle 
having a braking system in which hydraulically activated service brakes 
are applied or assisted by compressed air or vacuum must be equipped 
with both a warning signal that conforms to the requirements of 
paragraph (b) of this section and a warning device that conforms to the 
requirements of either paragraph (c) or paragraph (d) of this section.
    (f) Maintenance. The warning signals, devices, and gauges required 
by this section must be maintained in operative condition.

[37 FR 5251, Mar. 11, 1972, as amended at 53 FR 49400, Dec. 7, 1988]