Primary and Secondary Air Pressure Gauges
Warning: If air pressure falls below minimum pressure, the braking ability of the vehicle will be limited. Slow the vehicle down and bring it to a gradual stop. Do not attempt to move the vehicle until air pressure has risen above the minimum level. Moving a vehicle without adequate braking power could cause an accident resulting in property damage, personal injury, or death.
Air pressure gauges register the pressure in the primary and secondary air systems. Normal pressure with the engine running is 100 to 120 psi (690 to 827 kPa) in both systems. See Fig..
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Primary Air Pressure Gauge
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Secondary Air Pressure Gauge
Fig. 1, Air Pressure Gauges
Air pressure gauges are required on all vehicles with air brakes. A low-air-pressure warning lamp and buzzer, connected to both the primary and secondary systems, activate when air pressure in either system drops below a minimum pressure of 65 to 75 psi (448 to 517 kPa).
When the engine is started, the warning lamp and buzzer remain on until air pressure in both systems exceeds minimum pressure.