§ 393.51 Warning signals, air pressure and vacuum gauges.

(a) General Rule. InEvery thebus, mannertruck and totruck thetractor, extentexcept specifiedas provided in paragraphsparagraph (bf), (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) HydraulicThe brakes.warning Asignal vehiclemust manufacturedmeet onthe orapplicable afterrequirements Julyof 1paragraphs (b), 1973(c), and(d) havingor service(e) brakesof activatedthis bysection.
(b) hydraulicHydraulic fluidbrakes. mustVehicles bemanufactured equippedon withor aafter warningSeptember signal1, that1975, performsmust asmeet follows:
(1)the Ifbrake Federalsystem Motorindicator Vehiclelamp Safetyrequirements Standardof FMVSS No. 105 (§ 571.105 of this title(S5.3) was applicable to the vehicle aton the timedate itof wasmanufacture. Vehicles manufactured, theon warningor signalafter mustJuly conform1, to1973 thebut requirementsbefore ofSeptember that1, standard.
(2)1975, Ifor Federalto Motorwhich VehicleFMVSS Safety Standard No. 105 (§ 571.105) was not applicable toon the vehicledate atof themanufacture, timemust ithave wasa manufactured, the warning signal mustwhich becomeoperates 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 readilyeither visible within the driver's forward field of view or audible. The signal must be continuous. (NOTE: FMVSS No. 105 was applicable to trucks and buses from September 1, 1975 to October 12, 1976, and from September 1, 1983, to the present. FMVSS No. 105 was not applicable to trucks and buses manufactured between October 12, 1976, and September 1, 1983. Motor carriers have the option of equipping those vehicles to meet either the indicator lamp requirements of FMVSS No. 105, or visiblethe indicator lamp requirements specified in this paragraph for vehicles which were not subject to FMVSS No. 105 on the driverdate of manufacture.)
(c) Air brakes. A commercial motor vehicle (regardless of the date itof was manufacturedmanufacture) havingequipped with service brakes activated by compressed air (air-mechanical brakes) or a commercial motor vehicle towing a vehicle havingwith service brakes activated by compressed air (air-mechanical brakes) must be equipped, andwith perform,a aspressure follows:
(1)gauge Theand vehiclea mustwarning havesignal. aTrucks, lowtruck airtractors, pressureand warningbuses devicemanufactured thaton conformsor toafter theMarch requirements1, of1975, eithermust, paragraphat (c)(1)a (i)minimum, orhave (ii)a ofpressure thisgauge section.
(i)and Ifa Federalwarning Motorsignal Vehiclewhich Safetymeets Standardthe requirements of FMVSS No. 121 (§ 571S5.1211.4 offor thisthe title)pressure wasgauge applicableand toS5.1.5 thefor vehicle at the timewarning itsignal) wasapplicable manufactured,to the warningvehicle deviceon mustthe conformdate toof themanufacture requirements of thatthe standardvehicle.
(ii) IfPower Federalunits Motorto Vehiclewhich SafetyFMVSS Standard No. 121 (§ 571.121) was not applicable toon the vehicledate atof themanufacture timeof it was manufactured, the vehicle must havebe aequipped devicewith—
(1) thatA providespressure agauge, readilyvisible audibleto ora visibleperson continuousseated warningin to the drivernormal wheneverdriving theposition, pressurewhich ofindicates the compressed air pressure (in thekilopascals braking(kPa) systemor ispounds belowper asquare specifiedinch pressure,(psi)) whichavailable mustfor bebraking; atand
(2) leastA one-halfwarning ofsignal thethat compressoris governoraudible cutoutor pressure.
(2)visible Theto vehiclea mustperson havein athe pressurenormal gaugedriving whichposition indicatesand provides a continuous warning to the driver whenever the air pressure in poundsthe perservice squarereservoir inchsystem availableis forat braking379 kPa (55 psi) and below, or one-half of the compressor governor cutout pressure, whichever is less.
(d) Vacuum brakes. A commercial motor 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 devicevacuum thatgauge, providesvisible to a readilyperson audibleseated orin visiblethe continuousnormal warningdriving toposition, thewhich driverindicates whenever the vacuum (in themillimeters vehicle'sor supply reservoir is less than 8 inches of mercury) available for braking; and
(2) A vacuumwarning gaugesignal whichthat indicatesis audible or visible to a person in the normal driving position and provides a continuous warning to the driver whenever the vacuum in inchesthe ofvehicle's mercurysupply availablereservoir foris brakingless than 203 mm (8 inches) of mercury.
(e) Hydraulic brakes applied or assisted by air or vacuum. AEach vehicle havingequipped awith braking system in which hydraulically activated service brakes which are applied or assisted by compressed air or vacuum, and to which FMVSS No. 105 was not applicable on the date of manufacture, must be equipped with both a warning signal that conforms to the requirements of paragraph (b) of this section andfor athe warninghydraulic deviceportion thatof conforms to the requirementssystem; of either paragraph (c) of this section for the air assist/air applied portion; or paragraph (d) of this section for the vacuum assist/vacuum applied portion. This paragraph shall not be construed as requiring air pressure gauges or vacuum gauges, only warning signals.
(f) MaintenanceExceptions. The warningrules signals,in devicesparagraphs (c), (d) and gauges(e) requiredof by this section mustdo benot maintainedapply into operativeproperty condition.
[37carrying FRcommercial 5251,motor Mar.vehicles 11which have less than three axles and (1) were manufactured before July 1, 19721973, asand amended(2) athave 53a manufacturer's gross vehicle weight rating less than 4,536 kg (10,001 pounds).
[70 FR 4940048052, DecAug. 715, 19882005]