will not shut off, this indicates an open drain cock or a
failure in the brake system.
It is not compulsory but it is advisable that
vehicles be inspected to be sure that the air gauges are
properly connected. The primary system should be
connected to the green needle and secondary system to
red needle. This can easily be checked by charging the
air system, bleeding off the primary system (rear brake
reservoir) and the green needle should drop. If the
green needle does not drop the air lines on the gauge
should be changed. All chassis would be assured that
identification of both systems will be uniform.
Check the air gauge for accuracy. The simplest
way do do this is to compare the pressures
registered by the gauge over its normal pressure
range with the pressure registered by a test
gauge known to be accurate.
A gauge which loses its accuracy must be
replaced. The continued use of a dash gauge
showing an error of more than 35 kPa (5 psi) is
LOW PRESSURE INDICATOR;(LP-3 TYPE)
The low pressure indicator (Fig. 4) is a safety
device designed to give an automatic warning whenever
the air pressure in the primary or secondary air brake
system is below approximately 483 kPa (70 psi).
Operating as an air-controlled switch of an electrical
circuit, the low pressure indicator automatically sounds a
buzzer when the air pressure drops too low. The
warning will be both visible (light) and audible (buzzer).
The nominal pressure setting of 483 kPa (70 psi)
is subject to a tolerance of plus or minus 41 kPa (6 psi)
so that the actual operating pressure of the low pressure
indicator may vary between 524 kPa (76 psi) maximum
to 441 kPa (64 psi) minimum.
Fig. 4 Exterior View of Low Pressure Indicator
OPERATION (Fig. 5)
To describe the operation, we shall assume the
Low Pressure Indicator is set for 483 kPa (70 psi).
Setting of indicator is marked on a label on valve body.
When air pressure at supply port and under the
diaphragm is above 483 kPa (70 psi), electrical contacts
remain open because the force exerted by air pressure
underneath the diaphragm overcomes force exerted by
the spring above the diaphragm.
When air pressure below the diaphragm drops below
483 kPa (70 psi), the spring exerts a force which is
greater than the force exerted by the air pressure below
the diaphragm. This causes the piston to move and
allow the electrical contacts to close. This completes or
closes electrical circuit to warning device, warning driver
of low air pressure in the system.
Fig. 5 Cross Sectional View of Low Pressure Indicator
Every three months or after 40,000 km (25,000
miles), check electrical connections.
Every year or 160,000 km (100,000 miles),
perform SERVICE CHECKS. If diaphragm is ruptured,
replace complete assembly.
Operation of the low pressure indicator may be
checked with ignition switch on, then by reducing the
reservoir pressure and being sure that the contacts close
when the reservoir pressure
CTS-4079 - CHAPTER II
PRINTED IN UNITED STATES OF AMERICA