Bendix AD9 Air Dryer Servicing

Mechanics Hub - January 2, 2014
Bendix AD9 Air Dryer Servicing

Servicing the Bendix AD9 Air Dryer is straight forward and an essential part of the air brake system. School buses use the 121 air brake system which uses anti-compounding valves built in to resist brake component damage. Neglecting the maintenance of the air dryer will cause huge problems down the road.

The air dryer consists of a dessicant filtering material and a valve release mechanism to exhaust oil and moisture build up when the compressor reaches cut out pressure (between 120 and 130 p.s.i.). It's recommended to service the air dryer yearly depending on how clean the air system and how many hours have accumulated. One sure sign of required service is the a build up of oil at the air dryer unloader exhaust port. There is a test supplied by Bendix that determines if an air compressor is under warranty for excessive oil usage.

The AD9 dryer kits come with seals, a spin on desiccant filter cannister and the unloader kit. The unloader can be removed by itself and a new kit installed which consists of orings, spring, valve and seat. The dessicant filter is a threaded assy that is easily screwed on to the bottom of the dryer housing. In our in house maintenance program we service the air dryer every 2 years because of the low hours and mileage accumulated in our fleet. (sooner if there is a compressor oil problem)

Why Do Air Brake Systems Need an Air Dryer

The air dryer works very well to remove unwanted moisture from the air system. The efficiency level of the air dryer is only as good as the amount of maintenance implemented. Since air creates heat when it's compressed... moisture is produced which starts at the air compressor. The air entering the air compressor is filtered but comes from the atmosphere so it is naturally moist combining with the heat caused by the compressor.

Over time air compressors wear out and will eventually start to bypass engine oil into the air system. The air dryer dessicant filter will absorb the oil and moisture and servicing is an important step. Neglect will cause contamination into the air system. The air valves will gum up and corrode from moisture and oil. This combination will cause a lot of headaches especially in the winter where moisture develops at a much higher level. Daily draining of the air tanks will help alleviate the build of moisture in the tanks.

To aid the air dryer from freezing up they come with a heater at the bottom of the unloader. Any build up of moisture will cause the unloader to freeze up in the winter. There is a steady battery supply to the heater with the ignition on.

The air governor cuts out the compressor at a preset air pressure to not only give the air compressor a rest but also to unload the accumulation at the bottom of the air dryer. So looking at what the air dryer does for the air brake system it is the most important component in my view and servicing must be monitored to keep the air brake system clean and prevent air valve malfunction.