TRUCK SERVICE MANUAL
BRAKES-AIR
Only test gauges known to be accurate are to be
used for checking brake valve delivery pressures,
governor pressure settings and other tests. Test gauges
differ from ordinary dash gauges in respect to material
and workmanship. Due to these differences they are
more accurate over entire range and maintain their
accuracy over longer periods.
After initial starting of engine, if air gauge shows
that one circuit has no air pressure nor a noticeable
increase in air pressure within a reasonable length of
Fig. 4 Exterior View of Low Pressure Indicator
time (30 to 60 seconds) and the low pressure switch and
low pressure light will not shut off, this indicates an open
OPERATION (Fig. 5)
drain cock or a failure in the brake system.
To describe the operation, we shall assume the
NOTE: It is not compulsory but it is advisable that
Low Pressure Indicator is set for 70 psi. Setting of
vehicles be inspected to be sure that the air gauges
indicator is marked on a label on valve body. When air
are properly connected. The primary system should
pressure at supply port and under the diaphragm is
be connected to the green needle and secondary
above 70 psi, electrical contacts remain open because
system to red needle. This can easily be checked by
the force exerted by air pressure underneath the
charging the air system, bleeding off the primary
diaphragm overcomes force exerted by the spring above
system (rear brake reservoir) and the green needle
the diaphragm.
should drop. If the green needle does not drop the
When air pressure below the diaphragm drops
air lines on the gauge should be changed. Al
below 70 psi, the spring exerts a force which is greater
chassis would be assured that they are connected in
than the force exerted by the air pressure below the
the same manner and identification of both systems
diaphragm. This causes the piston to move and allow
will be uniform.
the electrical contacts to close. This completes or closes
electrical circuit to warning device, warning driver of low
SERVICE CHECKS
air pressure in the system .
1. Check the air gauge for accuracy. The simplest way
to do this is to compare the pressures registered by the
gauge over its normal pressure range with the pressure
registered by a test gauge know to be accurate.
2. A gauge which loses its accuracy must be replaced.
The continued use of a dash gauge showing an error of
more than 5 pounds is not recommended.
LOW PRESSURE INDICATOR (LP3 Type)
DESCRIPTION
The low pressure indicator (Fig. 4) is a safety
device designed to give an automatic warning whenever
Fig. 5
Cross Sectional View of Low Pressure Indicator
the air pressure in the primary or secondary air brake
system is below approximately 70 pounds. Operating as
MAINTENANCE
an air controlled switch of an electrical circuit, the low
Once a month or after 10,000 miles, check
pressure indicator automatically sounds a buzzer when
electrical connections.
the air pressure drops too low. The warning will be both
Every year or 100,000 miles, perform SERVICE
visible (light) and audible (buzzer).
CHECKS. If diaphragm is ruptured, replace complete
The nominal pressure setting of 70 pounds is
assembly.
subject to a tolerance of plus or minus 6 pounds so that
the actual operating pressure of the low pressure
indicator may vary between 76 pounds maximum and 64
pounds minimum.
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