Happy New Year! 2012 brings us another year full of opportunities to better our operations and make the most of our equipment. In a recent article we discussed the basics of how air brake systems work. I would like to pick up where we left off by discussing some of the problems you may encounter with your air brake system.

Schlund jim
Retired Diesel Mechanics Professor / College of Southern Idaho

The compressor is the part that builds up air pressure, which is then delivered throughout the system to supply tanks that are used for service brakes, emergency brakes and trailer brakes.

Because the compressor is lubricated and cooled by the engine, there is the possibility of oil and coolant leaking into the air system. This can cause damage to the compressor and the rest of the system.

If there is excessive oil passing through the compressor, this could mean the compressor’s rings have excessive wear and should be replaced. Another cause could be that the oil return line to the engine is blocked. Make sure this line is free of kinks or sharp bends.

If the compressor is making too much noise, you might check the compressor’s drive system. The drive gear or pulley could be loose and need tightening. The drive gear could also be worn and need to be replaced.


Another important point to remember is that air compressors need to have clean air going into them. The air that goes into the compressor usually has passed through the engine’s air cleaner. In some systems, the compressor has its own air cleaner. Replace these air cleaners if they aren’t filtering properly.

Air dryers
Once the air leaves the compressor, it goes through the air dryer, where all the impurities in the air are removed. However, the air dryer falls subject to many factors that can cause it to malfunction.

If working properly, the dryer should cycle on and off to exhaust impurities out of the system. If the dryer is constantly cycling, inspect the system for air leaks.

If you notice the dryer is not purging or exhausting air, check the line to the compressor to make sure it is in good shape and not plugged. A faulty purge valve can also cause the dryer to fail.

Make sure to service air dryers periodically to avoid having dirty air circulate through the system and end up in the supply tanks. In the winter, any moisture trapped in the system can cause freeze-ups. In the summertime, gunk and debris can clog up the system.

Some air dryers have desiccant cartridges that absorb moisture. These cartridges can become too contaminated to function properly and must then be replaced.

Reservoir (supply) tanks
The supply tanks throughout the air system must be monitored closely. Air must be available in the supply tanks in order for the brakes to be applied, which occurs with the help of relay valves.

Why do we need that reservoir air waiting in the supply tanks? Simply put – we don’t want delays. You do not want to wait for air pressure to build up and reach the supply tanks, especially if you need the brakes to work quickly. Therefore, supply tanks should always work as efficiently as possible.

Water accumulation can occur from time to time in reservoir tanks. Even in a good operational system, there’s still some level of condensation that occurs from the moisture in the air.

As the air is compressed, it heats up, causing any moisture to evaporate, pass through the compressor and air dryer and then condense in the supply tanks. Make sure to drain the supply tank if there is an accumulation of water.

If coolant is present inside the reservoir tank, check the compressor cooling system to identify and repair the problem.

System valves and brake lines
As you’ll remember, we said air brakes have a series of valves throughout the system that help regulate the air flow and keep air in the supply tanks. These valves are very sensitive to oil, moisture, dirt, rust and debris. Always repair or replace any valves that are faulty.

Once in a while, people at the parts store mix up your order and give you the wrong parts, such as truck and trailer relay valves. If these are confused, it would quickly cause an issue. Always check that the parts you order and receive are the parts you need.

Visually inspect air brake lines, especially if you notice a slow buildup of air pressure. These lines connect the air brake system together and can crack, chafe or become loose due to vibrations, all of which can cause air leaks.

The line from the compressor to the dryer, or hot line, can become plugged with carbon deposits. If this happens, remove the line, clean it or replace it. Replace any lines that have become damaged.

Know your system
The problem areas I have highlighted are just a few of the many that can affect your brakes. Consult with a technician if you aren’t sure of the cause of the problem.

Failures in air brake systems start when people don’t understand how the system works and don’t know how to service and maintain the system properly. It just makes sense to understand the system.

It is also equally important to have proper training before working on air brake systems. PD

Mechanic’s tip: Do not work on emergency parking brake canisters unless you have been properly trained. If mishandled, the springs inside these canisters can cause serious injuries.

See the first part of the series on air brake systems "Mechanics Corner: Understanding air brake systems."


Jim Schlund
Retired Diesel Mechanics Professor
College of Southern Idaho