Clean And Dry

The Chicago Rawhide Turbo-2000 air dryer is designed for use with turbo-charged air compressors and contains turbo boost even when in the purge cycle.

Water on the road is not the only moisture you need to worry about when you apply your brakes. Water can build up inside your air brake system and cause your brakes to partially or completely malfunction. A properly maintained air dryer protects the whole system by siphoning moisture and oil out of the air before they reach any working parts.

“ABS valves, relay valves and suspension leveling valves – anything that’s lubricated – is especially vulnerable,” says Fred Hoffman, engineering manager for air dryers at Bendix Commercial Vehicle Systems.

Leslie Kern, heavy-duty products manager at Chicago Rawhide, agrees. “ABS valves will not function properly if there is moisture present,” Kern says. “So, air dryer maintenance is more important now than it was before the advent of ABS.”

Water vapor turns to liquid inside the air brake system because it’s under pressure (90 psi to 120 psi). Unless separated from the air stream, the water collects in the air brake system tanks and piping and combines with oil from the compressor – which gets around the piston rings – and forms a light brown emulsion. This emulsion can clog and corrode brake system valves and air lines, which can lead to expensive maintenance and perhaps a serious brake system problem. In winter, the water can freeze – usually in the small orifices of a relay or ABS valve or at a bend or low point in an air line – and cause complete malfunction.

Air flows through the dryer in its trip from the compressor to the wet tank during the on cycle of the compressor. Air dryers contain a replaceable cartridge filled with a desiccant, a drying material. An oil separator pulls most of the oil out before it reaches the desiccant to keep it clean. The small beads in the desiccant hold the moisture in a process called adsorption. When the compressor shuts off, a purge valve opens and the pressure drops to normal outside air pressure. The pressure drop causes the moisture to be pulled off the desiccant and expelled through an exhaust port.

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The desiccant bed and oil separator have a limited life – normally about one to three years. The dryer’s internal workings would last forever except that oil contamination eventually works its way though the separator and keeps the beads from grabbing water the way they should.

Here are some key points in maintaining your dryer and the rest of the air brake system:

REPLACE THE DESICCANT CARTRIDGE AT THE RIGHT TIME. This is the most important part of dryer maintenance. Kern says Chicago Rawhide air dryer cartridges should be replaced every three years in linehaul service, two years in inner city, and every year in refuse and off-highway applications. CR sells its cartridges as part of service kits that also include the turbo cutoff valve and purge valve and recommends replacing all three at once.

DRAIN THE TANKS PERIODICALLY. Draining of the wet tank and primary and secondary storage tanks, says Larry Donaldson of Bendix, serves as a check of dryer function and should be done at least every 25,000 miles, 400 hours or three months, whichever comes first. Assuming the dryer is the right one for the application and the air system is in good condition, a lot of moisture in the tank (normally mixed with some oil) means it’s time to replace the cartridge. Some moisture is normal, but you should not have much if the dryer is working properly.

Jon Canale, an engineer at MeritorWabco, says it’s smart to do this as often as weekly. “When you drain air tanks, you don’t need to empty them. Just pull the cord and hold open for a few seconds so you can see what comes out,” Canale says. Completely draining the tanks interferes with dryer performance.

CHECK FOR OIL GETTING THROUGH THE CARTRIDGE. “If you see oil accumulating around the purge valve and just below where the exhaust port is, you should at least examine the cartridge,” Canale says. Unless the cartridge is overdue for maintenance, a heavy evidence of oil could be a sign of an incorrect installation. The problem would be an air line between the compressor and dryer that’s too short and not cooling the air stream sufficiently.

Frequent failure of dryer purge and turbo valves would be another symptom of this problem. Canale says to check the length of the air line from compressor to air dryer. With newer, cooler-running compressors, the length can be as short as 6 feet, but on older trucks, especially in vocational applications, it should be as long as 12 feet. Check the manual for the correct length. Also, inspect carefully for dips or loops and make sure there is a continuous, gentle downhill slope so moisture and freezing won’t block the line. If the dryer has performed without this problem for some time, but it has cropped up recently, the compressor could be worn out.

LISTEN FOR YOUR REGULAR PURGE CYCLE. That’s the blast of escaping air you can hear when the purge valve opens. Kern explains that the air compressor governor, also called an unloader, is supposed to cycle the compressor off and send a signal to the dryer to exhaust at 120 psi. After that, the brake system uses the air in the tanks until pressure drops to 90 psi. Then, it cuts back in. If the cycle occurs too frequently, your unloader may be failing and may need to be replaced, Kern says.

MONITOR YOUR COMPRESSOR CYCLES. Air system leaks can cause the compressor to come on after less off time and stay on longer. If a leak is big enough, it will stay on all the time. In this case, says Kern, “Figure out where the leak is. It could be in the dryer or anywhere in the system.”

To find a leak, run the engine to charge the system to the point where the governor cuts out, then listen to the purge of air from the dryer. There will be “a significant initial blast,” Canale says, indicating that the dryer is purging freely. Listen – but don’t feel – for this because you could get injured by debris. But, once the blast slows down, put your hand under the exhaust port. You should feel a slight flow for 15 to 30 seconds, after which it will shut off. If flow continues, the most likely cause is a failed turbo cutoff valve, assuming the air compressor is turbocharged.

You can quickly determine if you have this type of system by tracing the inlet line to the compressor. If it comes from the engine intake manifold, you have a turbocharged compressor. If it comes from upstream of the turbo compressor, and the turbo valve has failed, you’ll hear a chugging noise. In a few cases, leakage during the off cycle can result from a failed check valve on the dryer discharge line leading to the brake system.

Next, have someone dump system pressure by fanning the brakes. Soon, you’ll hear the compressor cut in. After the system charges back up to nearly the cutoff pressure, shut the engine off. Now feel for air escaping from the purge valve’s exhaust port. If there is leakage, the problem is a failed purge valve. Leakage elsewhere in the brake system, for example, on tank drain valve O-rings, will tax the compressor and dryer and should be fixed.

NOTE FREQUENT CARTRIDGE REPLACEMENTS. If you’ve had good service in the past but now you have to replace the cartridge often due to oil, it’s probably time to replace the compressor. If it’s worn, you’ll also hear it running more of the time even though the brake system is relatively free of air leaks.

NOTE DRYER HEATER PROBLEMS. The dryer has a heater for the exhaust port to prevent expelled moisture from freezing and stopping the purge function. But, obviously, a heater problem won’t show up until it’s cold outside.

Randy Nichols, air dryer engineer for the Bendix AD-9, says this heater most often fails because of “lack of voltage due to a broken wire, corroded wire or connection, or blown fuse. Put a voltmeter on the plug going into the heater. It’s a simple test that can be done in any weather and covers most failures.”

Bendix’ Larry Donaldson says if it’s cold outside, first turn off the truck to cool the dryer to below 40 degrees F. Then, unplug the connection to the heater and run an ohmmeter between the connector pins. With a standard 12-volt system, resistance should be 1.5 to 3 ohms. If the unit happens to operate at 24 volts, resistance should be 6 to 9 ohms. Otherwise, replace the heater. Consult the manual for your dryer model for exact resistance.

Chuck Eberling of Bendix recommends that when it’s cold out, take a minute to “cycle the brake pedal to vent the system to below cut in pressure right after shutting off the engine. If the engine is shut off with the dryer in the purge position, the dryer could freeze up, preventing the brake system from charging back up.”

Draining the air tanks to keep an eye on dryer performance and replacing the desiccant cartridge with its integral oil separator as necessary will keep your dryer functioning for a long time. Following up with the additional checks at least yearly will help you solve additional problems resulting from dryer component or air compressor wear. Clean, dry air will greatly prolong the life of critical brake system air components, helping to keep you safe and minimize your downtime.


For most trucks that are well-maintained and have a properly installed air dryer, replacing the cartridge at the right time will protect your brake system. Replacement is normally straightforward, but the procedure varies widely from dryer to dryer. You can often find manuals on manufacturer websites or see your dealer or distributor for printed copies.

Many aftermarket cartridge designs are cheaper than the $30 to $40 price of original parts, though the makers often substitute plastic for the metal used in Arvin-Meritor cartridges. Jon Canale, an engineer at MeritorWabco, believes this material isn’t strong enough to withstand the normal limits on air dryer heat and can fail, allowing beads to clog your system – a mess that’s costly to cure. On all units, be sure to wipe out any accumulated oil and moisture; a few units have an integral sump to help collect them.

When replacing the cartridge, first drain all air from the system so it will be safe to disassemble parts that normally hold pressure. For example:

  • On the Bendix AD-9, eight bolts (two of which fasten the unit in place) must be removed so the outer shell can be lifted off the base. The cartridge is then unscrewed with a strap wrench, its two O-rings changed, and the unit reassembled.
  • On the Bendix AD-IP, removing a through-bolt from underneath the dryer base allows the cartridge, which slightly resembles a spin-on oil filter, to be lifted off.
  • On the MeritorWabco System Saver 1200 and the Chicago Rawhide Turbo-2000 and Turbo-3000, the cartridge is spun off and on much the way you change a spin-on oil filter. On the Chicago Rawhide units, an O-ring needs to be replaced.