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Posted: August 1, 2014 by Rolf Lockwood

Cold weather. Frozen air lines. Major headaches. The life of a trucker in winter.
You can’t avoid the thermometer’s tale, but you sure can do something about the headaches. At least about the ones caused by frozen and blocked air lines.

The solution starts with a spec’ing decision, of course, namely choosing an air
dryer. There can’t be many Canadian trucks sent into the world without one, but
just in case you need convincing, listen to what the folks at Bendix have to say.

They tell us that the compressed air inside a brake system unprotected by an air
dryer is always at 100% humidity. Even a one-degree drop in temperature will
cause condensation to occur, they say, and not just in the reservoirs, but in the
lines and valves throughout the system.

Even a very small amount of water, less than a teaspoon, can freeze and disable a portion of an air brake system. At highway speeds with the temperature at 25° F
or -4 °C, that can happen in less than 15 minutes. And an ice crystal the size of a
pinhead can cause an air leak large enough to drain the air from an average-size
service reservoir in less than 10 minutes.

Another interesting fact: a heavy truck’s air compressor puts out 4-6 ounces of
water during an average day’s operation.

So you’re going to have moisture, and if you’re not careful it’s going to freeze.
An air dryer will remove most of that moisture before it reaches the wet (supply)
tank. As warm, moist air from the compressor hits the cool outer walls of the dryer,
some moisture will condense on the walls and drain down into the purge-valve area. The air is then forced through a filter medium that will remove more moisture, and the treated air then exits the dryer through a one-way check valve.

As the governor periodically signals the air compressor to stop pumping, it also
signals the purge valve on the air dryer to open and exhaust all the air it contains, along with collected water and other contaminants. This purge cycle also results in
a reverse flush action that helps clean the filter material.

Those other air-system contaminants – like oil, perhaps from a worn compressor – will thicken and can cause malfunctions that never appear during warmer
seasons. The result can be sticky or even non-operative brakes.

So twice a year or so, even if you have an air dryer, the whole system needs a
cleanout. Some basic cleaning procedures are ones you can do yourself, or make sure that your mechanic does. Without going into too much detail, this is what has
to be done:

1. Disconnect and/or disassemble air lines, valves, and other components;

2. Blow out these components with clean air from the shop compressor, usually in the opposite direction to the system’s normal air flow (i.e., backwards); and

3. Inspect all parts and replace them if necessary.
If you find oil or other contaminants, you’ll then have to find the source and fix the problem. Oil usually comes from the air compressor, which may need repairs. Dirt could mean the truck’s air cleaner element needs to be changed. Particles of rubber come from somewhere in the system, and must be traced.

Excessive moisture comes from a non-working dryer.

If you find yourself needing to replace valves and air lines, it’s vital that any new parts be of the exact same specs as those being replaced. Each replacement
valve, for example, should be the same make and model as the old one, so that the crack pressure (the point at which the valve opens) stays the same. Each new air line should be of the same diameter and fittings should be of the same type
(connectors, unions, elbows, and tees) as the old, so the same volume of air is

If you ignore this point, brake performance will change. You’ll probably throw off brake balance on all your axles, and perhaps even between brakes on the same
axle. And you’ll almost definitely affect brake timing, for the worse, between the tractor and the trailer.

Here are some other tips to help you avoid air-system freezeup in winter:

1. Avoid idling your engine for long periods, because you’ll likely cause
compressor discharge-line freeze-up and maybe fry the compressor. Bendix warns that the discharge line should slope downward from the compressor discharge port to the supply reservoir without forming water traps, kinks or

2. Check the entire brake system for excessive air leakage, which causes the
compressor to pump more air and thus more moisture into the brake system.

3. Daily reservoir draining is the most basic step in limiting freeze-ups, and that
means all reservoirs because they all accumulate water and contamination.

Bendix says you must turn the engine off and drain all air from the reservoir. Better
still, open the drain cocks on all reservoirs and leave them open overnight to
ensure that all contamination is drained. If you’ve got automatic drain valves,
make sure they’re working.

As always, the best maintenance is the preventive kind.


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