AIR BRAKES - GENERAL INFORMATION
Multiple Trailers
The Pressure Balance and Pressure Build-up Rate Test
also can be used for checking combination vehicles with
multiple trailers. One duplex test gauge is installed as
described in the test procedure to read brake pressures
at the rearmost axles of the tractor and the first trailer. A
second duplex test gauge is installed to read pressure at
the dolly axle and the rearmost axle of the second trailer
(Figure 30). Two additional air hoses may be required to
make the gauge connections at the second trailer in
order to keep both duplex gauges at one observation
point. The test is then conducted as described, noting
and recording the pressure differentials and build-up
times obtained from the four sources - the rearmost
axles of the tractor and first trailer, and the dolly axle and
rearmost axle of the second trailer.
Multiple-trailer combinations and their related complex-
ity make it difficult to establish positive criteria. However,
meeting the following guidelines should help optimize
performance. For the pressure balance check, the
average pressure difference between axles through the
pressure range tested should fall within 28 kPa (4 psi) for
optimum performance.
For the pressure build-up rate check, the maximum
pressure difference (lead or lag) between axles should
normally be no greater than 103 kPa (15 psi) before the
pressures equalize.
Again, if the pressure balance and/or pressure build-up
rate differences exceed the acceptable limit(s), refer to
the Data Interpretation and Corrective Measures section.
Torque Balance Test
NOTE - If the pressure difference recorded for the
pressure balance check exceeds the acceptable limit,
the Imbalance must be corrected before a Torque
Balance Test Is performed. However, if the pressure
build-up rate indicates a problem condition, the Torque
Balance Test can and should be conducted, especially if
the operator’s basic concern Is that of uneven lining wear
between tractor and trailer.
Figure 30Test Connections Multiple Trailers
CTS-4154Y - Page 31
