Source: https://cms.fmcsa.dot.gov/print/regulations/title49/section/393.51
Timestamp: 2019-04-19 21:28:10+00:00

Document:
Lamps and reflectors—combinations in driveaway-towaway operation.
Requirements for head lamps, auxiliary driving lamps and front fog lamps.
Requirements for lamps other than head lamps.
Brakes required on all wheels.
Warning signals, air pressure and vacuum gauges.
Truck and truck tractor window construction.
Coupling devices and towing methods, except for driveaway-towaway operations.
Coupling devices and towing methods, driveaway-towaway operations.
Warning flags on projecting loads.
Seats, seat belt assemblies, and seat belt assembly anchorages.
Which types of commercial motor vehicles are subject to the cargo securement standards of this subpart, and what general requirements apply?
What are the minimum performance criteria for cargo securement devices and systems?
What are the general requirements for securing articles of cargo?
Section § 393.51: Warning signals, air pressure and vacuum gauges.
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Question 1: Is the low pressure warning device required to activate before the tractor protection valve?
Guidance: No. §393.51  does not explicitly require the warning device to operate before the protection valve. It is implied that if the operating pressure of the warning device is at least 1/2 of the governor cut-out pressure, and that pressure is not less than the pressure at which the protection valve (or similar device) activates, the requirements of §393.51  are satisfied.
Question 2: Is the vacuum portion of vacuum-assisted hydraulic brake systems required to have a warning device?
Guidance: No. Only the hydraulic portion of vacuum-assisted hydraulic brake systems is required to have a warning device. Federal Motor Vehicle Safety Standards (developed and issued by the NationalHighway Traffic Safety Administration) (FMVSS) No. 105 does not require a warning device for the vacuum portion of the vacuum-assisted hydraulic brake systems. It is the intention of the Federal Highway Administration (FHWA) that §393.51  be consistent with Federal Motor Vehicle Safety Standards (developed and issued by the NationalHighway Traffic Safety Administration) (FMVSS) No. 105.
Question 3: Are vacuum gauges required on the vacuum portion of vacuum-assisted hydraulic brakes?
Guidance: No. §393.51(d)(2)  requires only that Commercial Motor Vehicle CMVs with vacuum brakes (not hydraulic brakes applied or assisted by vacuum) be equipped with a vacuum gauge.
Question 4: Is a warning device required in a Commercial Motor Vehicle CMVs with a single hydraulic brake system which uses the driveline parking brake as the emergency brake system?
Guidance: No. Warning devices are not required on such Commercial Motor Vehicle CMVs CMVs because the driver will be given ample warning of system failure by the movement and feel of the brake pedal.
Question 5: What difference, if any, is there between a warning device and a warning signal?
Guidance: For purposes of §393.51 , the terms may be used interchangeably.

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