AIR BRAKES - GENERAL INFORMATION
respectively, as shown in Figure 28. As each pressure
increment is reached at the tractor’s rearmost axle, note
and record the pressure reading for the trailer’s rearmost
axle. Repeat this step three times.
IMPORTANT - When applying the brakes, air pressure
must be Increased steadily and maintained at each step.
Any slight decrease In pressure will affect the accuracy
of the gauge readings and must be avoided. Note and
record the pressure only after all gauge recordings are
stabilized.
Figure 27 - Test Gauge Hose Connection
(At Service Brake Chamber)
Figure 28 - Brake Test Application Pressures
Under normal conditions, the average pressure vari-ance
between axles through the pressure range as test-ed
should not exceed 28 kPa (4 psi) for optimum
performance. (See Data Interpretation and Corrective
Measures.)
4.
Check pressure build-up rate as follows: While
observing the duplex gauge, apply the service
brake control moderately to at least 207 kPa (30
psi). The moderate rate of application should
simulate that used in day-to-day braking under
normal operating conditions. Observe and
record the pressure difference between the
hands on the test gauge dial (Figure 29). The
dual pressure readings give an indication of the
timing difference between the axles. If the
pressure build-up in the trailer brakes leads the
tractor brakes, the gauge reading should be
recorded as a positive (+) number. If the
pressure build-up lags, the reading should be re-
corded as a negative (-) number. Release the
service brakes. Repeat this step at least three
times and record the average of the total
readings.
Figure 29 - Gauge Reading - Pressure
Balance Check
Initial pressure difference (lead or lag) between
axles should normally be no greater than 69 kPa
(10 psi) for single-trailer combinations before the
pressures begin to equalize.
NOTE - Since pressure lead and lag derived from using
a duplex test gauge are estimates only, pressure build-
up rate tests involving large fleets of vehicles can be
performed more easily and accurately using pneumatic
timing devices. These Instruments are available from
electronic laboratories and air valve manufacturers. The
above check, however, will provide an indication if slow
pressure build-up rate Is a cause for compatibility
complaints.
5.
If pressure balance and/or pressure build-up rate
differences
between
axles
exceed
the
acceptable
limitis1.
refer
to
the
Data
Interpretation and Corrective Measures section.
CTS-4154Y - Page 30
