suds. Leakage in excess of a 77 mm (3") soap
bubble in 3 seconds is not permissible.
Leakage is caused by dirty or scored key or
body. Leakage due to dirt is corrected by
cleaning parts and applying a thin coating of Item
1 in LUBRICANT SPECIFICATIONS on the key.
Leakage due to a scored key or body cannot be
repaired and the drain cock must be replaced.
The purpose of the safety valve is to protect the air brake
system against excessive air pressure. Should the air
pressure in the air brake system rise above the setting of
the safety valve at 934 kPa (150 psi), the valve opens
and permits pressure above 934 kPa (150 psi) to be
exhausted. It is located on the supply reservoir.
The safety valve is a piston type valve (Fig. 2). The
piston is equipped with an O-ring type seal which seats in
the body of the valve.
Fig. 2 Sectional View of Piston Type Safety Valve
Once each year or every 160,000 km (100,000
miles), the safety valve should be removed and
Leakage at the piston in the body should not exceed a 77
mm (3 in.) soap bubble in 3 seconds. If air leakage is
excessive the valve must be replaced since it is only
serviced as a complete component.
The safety valve is preset to "blow off" at approximately
934 kPa (150 psi). Since the safety valve must be
removed to perform any adjustments, it is suggested that
the valve be replaced when any defect is detected.
When replacing the safety valve be sure
to drain all air from the supply reservoir,
to prevent bodily injury when the valve is
removed. Draining of the primary and
secondary reservoirs is not required
since they are protected by check
AIR PRESSURE GAUGE
The purpose of the air pressure gauge is to
register the amount of reservoir air pressure in the air
system. While air pressure gauges of this type are
commercially accurate, they must never be confused
with or substituted for test air gauges, which are intended
primarily for accurately checking air pressure in the air
The air pressure gauge may be either two
gauges or a single gauge with two indicators as
illustrated (Fig. 3).
An air pressure gauge is located in both the
primary and secondary brake systems so that the actual
air pressure in both systems is indicated to the operator
of the vehicle.
Fig. 3 Air Pressure Gauge
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 time (30 to 60
seconds) and the low pressure switch and low pressure
CTS-4079 - CHAPTER II -
PRINTED IN UNITED STATES OF AMERICA