Your brake system has powerful allies and equally powerful adversaries. Its worst enemy is moisture, while the air dryer is its best friend.
Water is always forming inside your air brake system. The water vapor in outside air turns to liquid because of the pressure (90-120 psi). There’s also oil from the air compressor. The brake system uses the engine’s oil for lubrication, and some of that lube oil goes by the compressor’s piston rings and into the air headed for the wet air storage tank.
Unless separated from the air stream, the water lays in the air brake system tanks and piping and combines with the oil to form a light brown emulsion. Fred Hoffman, engineering manager for air dryers at Bendix Commercial Vehicle Systems, says, “The system is designed for air and not liquid. The air dryer protects the whole system. ABS valves, relay valves and suspension leveling valves – anything that’s lubricated is especially vulnerable.” Leslie Kern, heavy-duty product manager at Chicago Rawhide, agrees, saying, “ABS valves will not function properly if there is moisture present. So air dryer maintenance is more important now than it was before the advent of ABS.”
The emulsion can clog and corrode those brake system valves or even air lines, leading to expensive maintenance, even a serious brake problem while you’re driving. In winter, it can also freeze – usually in the small orifices of a relay or ABS valve or at a bend or low point in an air line.
Installing and maintaining a good air dryer prevents such trouble because it gets the moisture and oil out of the air stream before it reaches any of the working parts.
The dryer’s internal workings could last forever if oil contamination didn’t eventually work its way though the separator and keep the beads from grabbing water the way they should. The desiccant bed and oil separator have a limited life – normally about one to three years, though in ideal applications it may be longer. Because of this, both are assembled into the dryer together as a replaceable cartridge. The most important single part of dryer maintenance is replacing that cartridge at the right time. As with almost everything in trucking, the duty cycle of the truck greatly affects cartridge life. Brake air system maintenance also plays a big role because leaks mean the compressor handles more air, meaning more water for the dryer to take out.
The most basic air dryer maintenance is to drain the wet tank and primary and secondary storage tanks periodically. This, says Larry Donaldson of the Bendix Tech Team, 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. Jon Canale, a senior project engineer at MeritorWABCO, says it’s smart to do this as often as once a week.
If the dryer is the right one for the application and the air system is in reasonable condition, a lot of moisture in the tank (normally mixed with some oil) means it’s time to replace the cartridge. “It’s normal to see a tan-colored goo in the air stream that some describe as ‘mayonnaise,’” Canale says. “Some is normal, but you should not have much if the dryer is working properly. You’ll just see a little mix in the initial blast of air.” In ideal applications, there won’t be any.
When you drain air tanks, you don’t have 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, putting some water downstream till things stabilize.
You should examine the cartridge if you see oil accumulating around the purge valve and just below the exhaust port, Canale says. Heavy evidence of oil there can be a sign of an incorrect dryer installation that has allowed heat to build up. The Meritor WABCO tape “System Saver Series Single Cartridge Air Dryers” says you may actually find the turbo cutoff valve to have a hole burned right through the center. The dryer purge valve may also
fail, and the valve failures will often be accompanied by oil downstream of the dryer as well as frequent cycling of the air compressor. Check the length of the air line from compressor to dryer “to make sure it’s cooling the air properly,” says Canale. The length with newer, cooler running compressors can be as short as 6 feet, but on older trucks, especially in vocational applications, it should be longer – as long as 12 feet. Check the manual for the correct specification, and make sure the line is neither too short nor too long. Also inspect carefully for “dips or loops,” which are bad news and need to be straightened out. There should be a continuous, gentle downhill slope so moisture and freezing won’t block the line.
If there is also oil in the air tanks and bottom of the dryer, your cartridge definitely isn’t working the way it should be. The cartridge is saturated with oil, and it’s passing right through. If the problem crops up after months of satisfactory operation, change the cartridge.
Leslie Kern says CR air dryers should have the cartridge replaced every three years in linehaul service, two years in inner city and every year in refuse and off-highway applications, where air usage is highest. CR sells its cartridges as part of service kits that also include the turbo cutoff valve and purge valve and recommends you replace all three at once.
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