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. 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 FHWA that §393.51 be consistent with 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 CMVs with vacuum brakes (not hydraulic brakes
