Air Brake Manual

Air Pressure Gauge
Vehicles with an air brake system are equipped with
a reservoir air pressure gauge (29). This gauge is
mounted in the cab, usually on the dashboard and
indicates the air pressure in the primary and
secondary or dry reservoirs. The supply or wet
reservoir does not usually have an air pressure
gauge. Common operating pressures are 80 to 135
psi, depending on the system. Monitoring the gauge
will alert the driver to any unusual changes in air
pressure.
Brake Application Gauge
An additional gauge can be installed on the dash to
indicate the application air pressure when the
brakes are applied. This gauge can be piped to
indicate the pressure of either a foot or hand
application. (Hand application will be explained later
in the manual.)
Low Pressure Warning Device
All vehicles equipped with an air brake system must
have a device to warn the driver if the air pressure in
the system drops to a dangerous level. This device
must be comprised of two systems - visual and audible
- consisting of a red warning light and a buzzer or a
wig wag. Due to overuse or leaks, the low pressure
indicator switch (9) will turn on a red warning light on
the dash or cause a buzzer to sound at or before 55
psi. Some vehicles are equipped with both a light and
a buzzer to warn the driver of a low air pressure
condition.
Wig-wags are not found in modern vehicles having
been replaced with a red warning light and buzzer.
They may still be in use on older vehicles. There are
two types of wig-wag low pressure warning devices
that may be used. Both types will drop into the driver’s
view should the system pressure drop to 55 psi. The
automatic warning device will rise out of the driver’s
view when the pressure in the system rises above 55
psi. The manual reset type must be placed in the “out
of view” position manually and will not stay in place
until the pressure in the system goes above 55 psi.
Whichever warning system is used, buzzer-lights or
wig-wag, the driver must stop the vehicle and find the
cause of the air loss. The air pressure remaining in the
system (approximately 55 psi) is enough for a brake
application if the driver acts promptly.
Stop Light Switch
Any driver following your vehicle must be warned
when reducing speed or stopping the vehicle. The stop
light switch (25) is an air-operated electric switch that
turns on the brake lights at the rear of the vehicle
when a brake application is being made.
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