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3 Reasons a Brake Line Air Dryer Won't Purge

Air tanks for brake system
Virtually all air contains a certain amount of water vapor, which can cause problems when it gets into the supply lines and tanks of an air brake system. To prevent corrosion, freeze-ups, and other moisture-related problems, most air brakes contain a component known as a dryer. An air dryer removes both water and oil from the supply air, thus keeping problems at bay.
Each time the brakes' compression cycle ends, the air dryer purges its accumulated moisture through a special valve. Sometimes, however, an air dryer fails to purge. As a result, the dryer soon reaches capacity and ceases to remove moisture from the air. This article takes a closer look at three common causes of an air dryer that won't purge.

1. Malfunctioning Governor

An air brake system uses a compressor to generate the air pressure necessary for safe braking. The system stores this pressurized air inside of a supply tank. As the compressor runs, the pressure in the supply tank increases. When air pressure reaches the appropriate level, a monitoring component known as the governor shuts down the compressor.
The governor also controls the behavior of the air dryer. So long as the compressor runs, the air dryer remains in its cycle mode, allowing air to flow through its filter cartridge and on to the supply tank. At the same time that the governor shuts off the air compressor, it also tells the air dryer to enter its purge mode.
Unfortunately, over time the governor may cease to send the appropriate signals to either the compressor or the air dryer. Such a failure often stems from mechanical issues inside of the governor itself, which regulates both the compressor and the air dryer by means of an unloader port signal.
If the system of pistons and springs inside of the governor breaks down, the air dryer may fail to receive the unloader port signal. As a result, the dryer won't enter purge mode, instead remaining perpetually in its cycle mode. Water quickly builds up, and the dryer reaches capacity. The only real solution here involves hiring a professional to install a new governor.

2. Governor Line Restriction

The governor's unloader port sends its signal by means of air pressure. One line leads to the compressor, telling it to shut off, while another line leads to the air dryer, telling it to enter the purge cycle. Even if the governor remains working correctly, the air dryer won't enter its purge cycle if the signal doesn't make it through.
Such a failure may occur as the result of restrictions in the unloader port line leading to the air dryer. Debris or ice may obstruct the free flow of pressurized air. Likewise, damage to the line may prevent the signal from reaching the air dryer. Have a trained professional inspect the lines leading to your air dryer to ensure that they remain in good working condition.

3. Oil Build-Up in Purge Valve

As noted above, the air dryer doesn't just remove moisture from your brakes' air - it also removes oil. Oil often gets into the air as the result of degraded seals or piston rings in the compressor. Even a perfectly maintained compressor allows small amounts of oil to leak through.
This oil accumulates in a special section of the air dryer cartridge. Eventually this oil separator reaches full capacity and can no longer accept new oil. At that point, oil begins moving through the filtration cartridge. Excessive amounts of oil on or around your purge valve indicates that your cartridge has reached the end of its lifespan.
Those who fail to replace their cartridge in a timely manner may eventually find that it no longer works at all. All of the oil accumulated at the bottom may block water from moving out the purge valve at all. For more information about what it takes to keep your air dryer working correctly, please contact the truck experts at S&T Truck Repair.