Ford F650 Owner's Manual page 32

Instrument Cluster
Do not drive the vehicle when the low air pressure buzzer is
sounding or the warning light is lit. These warnings indicate
there is not enough air pressure for the brake or suspension system to
operate properly.
Vehicles equipped with hydraulic brakes and an air compressor have a
single-pointer air gauge.
Note: This system does not have a low air pressure warning buzzer or a
low air pressure warning light.
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