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Understanding the air dryers now available in new brake systems

By on September 1, 2013

Can we agree that the brake system is a vitally important part of your truck? Certainly you want to feel confident that your brakes are working at peak performance.

Which brings us to the air dryer, which filters out contaminants like water and oil vapors that can interfere with peak performance. Manufacturers are working diligently to develop ways to more efficiently dry and condition the air in the brake system, and air dryers have been evolving to keep pace with those changes. These new air dryers do not look or sound the way you expect because they function differently. So let’s look at three very common air dryers for Class 8 trucks to see how each works and identify the keys to maximizing their longevity.

Bendix AD-9

The AD-9 is still popular, but its size and bulky footprint make it much less attractive for a manufacturer to place under the hood of the aerodynamic truck chassis we see today. This dryer has a replaceable desiccant cartridge; a serviceable purge valve, a turbo cut-off valve, and a check valve to prevent it from leaking during a purge cycle. If air is leaking from the exhaust of a brake valve, it is not always an indication that the brake valve is defective. This principle applies to the air dryer as well. Because the air governor, air compressor and air wet tank are connected to this air dryer, a malfunction in either direction or component can cause the AD-9 to leak from its exhaust. In addition, a failing air compressor can send contaminants into the air dryer, which will shorten its life and may prematurely wear or swell O-rings. The good news is there are service kits available for this dryer if it is truly defective.

Remember to replace the desiccant cartridge based on the manufacturer’s recommendation, which is normally every two years or sooner if you operate in humid and dusty environments.

Meritor System Saver 1200

This dryer very effectively cleans and dries the air before it gets into the brake system, helping to prevent damage to critical brake components. The System Saver 1200 is easily misunderstood. It typically does not have a turbo cut-off valve, which most operators have come to know on the AD-9.

A turbo cut-off valve prevents the truck’s engine from losing turbo boost each time the dryer goes into a purge mode. The exhaust is open during purge mode, allowing pressure to escape and resulting in an engine power loss if a cut-off valve is not used. But when a System Saver 1200 is installed on an engine that naturally aspirates the air compressor (meaning the air compressor does not pick up its air from the turbo charged side of the intake) there is no need for a turbo cut-off valve. This leaves the exhaust of the dryer open and exposed to the cycling of the air compressor during the unloaded/purge cycle.

You will hear air hissing from the exhaust of the air dryer but you will not notice your air gauges moving down, nor will your air dryer be cycling rapidly. This leaking of air can be misunderstood as an air leak, but it is a normal condition. When the air compressor is unloaded the pistons are just moving air back and forth. Because there is no turbo cut-off valve in the dryer, the air from the pistons cycling will escape from the dryer’s exhaust port. Do not try to repair the dryer because this is a normal condition.


These dryers are mounted directly to an air tank or a device that looks like a flying saucer. These integrated dryer systems contain multiple components, including pressure protection valves, purge valve and a number of charging valve components. They are designed to prevent the necessity of daily draining of air reservoirs.

These dryers have an easy-to-replace spin-on desiccant cartridge saving time and money. All replaceable assemblies can be serviced without removal of the air dryer and reservoir system from the vehicle. These dryers do have a turbo cut-off feature, which must be considered if you have an intermittent engine power complaint.

Get to know which air dryer you have on your vehicle and service it according to the manufacturer’s recommendation. After all, you don’t want your brake system to be merely adequate, you want it always to be working at top condition.

Homer Hogg, Technical Training Manager for TA and Petro, has worked as a truck technician for more than 30 years. He is ASE Master-certified, a Daimler Certified Trainer and a member of the Nashville Auto Diesel College Hall of Fame.

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