DIGGING IN

In Gear
checks and making sure the two air brake
systems remain completely separate, it will
never be caught,” says Mello. “If the problem
remains undiscovered and one system
loses pressure, the other will go down with
it rather than remaining at full pressure.
That’s the point of isolating the two systems
and keeping them independent.”
Air line routing should be inspected regularly for chafing and crimping.
Air lines on the tractor frame are often bundled into harnesses where chafing can occur.
Chafed air lines are an Out of Service condition.
protection valves found in the air dryer to
isolate the primary from the secondary.
Double or two-way check valves are
also used throughout the brake system
to keep the primary and secondary systems
separate and independent from one
another. Additional pressure protection
valves are used to keep auxiliary devices
such as air suspensions and air horns
from robbing all the air pressure from the
brakes during a failure.
“Roadside inspectors can, and sometimes
will, check these failure modes by
independently draining each reservoir,
making sure the other reservoir maintains
air,” Mello says. “During these single
failures the driver will be instructed to
make a service brake application, using
the foot valve, and the inspector will
check that at least one wheel or axle has
executed a brake application.”
Mello advises all heavy-duty maintenance
facilities to execute these failure
mode tests at least once a year, or at regular
Preventive Maintenance intervals.
“If the mechanics are not doing these
It starts with the compressor
Compressing moist, humid air creates
water in the air reservoirs that have to be
drained periodically – daily, in fact – as
per most manufacturers’ instructions.
However, it’s very difficult for drivers to
reach the air tank drain cables on today’s
aero-festooned tractors. That means
there’s a high likelihood of water making
its way past the wet tank and further
into the air system. While an air dryer
will solve some of that problem, older air
dryers with contaminated or oil-soaked
desiccant cartridges often are not able to
remove enough moisture from the compressed
air to prevent significant amounts
of water from entering the wet tank.
And since older compressors are also
more likely to throw oil, you now have an
oil-water mix to contend with, and that
can foul the extremely small orifices in
some brake system valves, like the expensive
ABS valve, and more.
“The extremely tight tolerances on
many of the pneumatic servos and solenoids
in automated transmissions and
the diesel exhaust delivery systems are
very sensitive to contamination,” notes
Richard Nagel, director of marketing and
customer solutions – air charging, Bendix
Commercial Vehicle Systems. “Oil in the
air tank is the canary in the coal mine.
Regular inspection of the exhaust air
from the air dryer and the wet tank discharge
is a critical maintenance item.”
Most of the air system problems resulting
from valve failures can be traced back
to contamination of some kind, usually
oil, water or dirt, all of which come
ultimately from the compressor, except
for possible dirt entering through open
gladhand ports.
Air systems are generally pretty
reliable and mostly trouble-free, but
the potential for problems still exists.
That makes the case for air system
maintenance. TT
48 TODAY’S TRUCKING