Saturday, September 22, 2007

Bus & Truck Air Brakes System-Bendix AD-9 Air Dryer


The Bendix AD-9 air dryer is easily serviced by disassembling it from the frame and removing the bottom cover bolts. The unloader located on the bottom housing can be removed and a kit installed. The kit comes with new o-rings, spring, valve and seat. The main filter which is filled with dessicant absorbs the oil and moisture that accumulates in the air brake system.
Since the air compressor creates heat this causes moisture to build up, so this is why air brake systems have an air governor that unloads the compressor at 120 p.s.i. and signals the dryer to dump the build-up of water & oil in the bottom housing to atmosphere. That's the quick air release you hear when building up the air system, the air guage should be reading a minimum of 120 p.s.i.
The dessicant filter has a threaded stud attached so it can be easily removed during a service. The Bendix AD-9 is a very reliable component and is usually trouble free but if you notice a lot of oil coming from the bottom housing exhaust port then that's telling you the compressor is pumping out more oil than it should. Bendix has a splash card that you hold under the exhaust port, that gives an indication of how bad the oil loss from the compressor really is. The next step is to replace the compressor and of course, service the air dryer. Under normal conditions the air dryer can be serviced every 1-2 years depending on the condition of the air compressor and highway miles or hours of operation.

The Bendix Air Dryer also have a built-in heater wired up to sense cold weather and keep the unloader valve from freezing up. Every Truck & Bus air system has an air dryer installed from the factory, it's the most important preventive maintenance item and should not be neglected.

14 comments :

Anonymous said...

Hello, I am here at Utah transit Authority. I am currently working on a AD9 and have a problem with air purging from the safety pop off valve. It seems to alternate with the air dryer purging. Air dryer will purge, and then air will start coming out of the safety pop off valve and then back again. Other mechanics here have worked on it and its becoming a little annoying. So far to date on its work history. The safety pop off valve, purge valve, air governor and the one way check valve have all been replaced. Any ideas or thoughts you can share with us???

Anonymous said...

Update,
Me again.... it appears I may of found a cause to our problem here. With the help of bendix.com, I was able to identify 2 systems that the AD9 air dryer can be found in. The first is the standard system and the other is the DLU system. It appears that our purchasing department has been purchasing the standard purge valve for our DLU system. Basically, we need the DLU purge valve assembly. I will keep giving updates as the journey continues.

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Anonymous said...

Hello, I need to replace bushings on the front leaf springs of a 99' international RE school bus. Do I need to take the springs completly off and use a press or is it easy enough to pound them out with the springs partially attached?

John Whelan said...

First question about the AD9. I have had the same problem and there is a hard seat and soft seat option on the unloader.

Spring bushings are much easier to replace on the hydraulic press. Removing the springs is a good idea

John Whelan said...

First question about the AD9. I have had the same problem and there is a hard seat and soft seat option on the unloader.

Spring bushings are much easier to replace on the hydraulic press. Removing the springs is a good idea

Frank said...

John's correct, press is easiest. you can however knock them out with an air hammer while they're still on the bus. good luck

ccrking said...

As a newbie to Bus and air brakes I have noticed that the air dryer seems to release air @ 120 PSI in a slow 2-3 seconds release versus a quick crisp purge. I was wondering if this is normal? It is a 1992 Thomas safe t liner. Does this use the Bendix AD-9 air dryer?

John Whelan said...

Your purge valve may be sticking and require a service. The exhaust process should be a quick operation.
Your bus will either have (besides the AD9) a Bendix AD-4 or a Midland Dryer assembly.

Anonymous said...

AD-9 air dryers as well as adis,when purging, large gust of air is expelled from dryer,if in dusty or gravel surface,it may create dust and debris to cause air filters as well safety.if you have had this problem,the fix is a AIR-DEFLECTER that is very easy to install.made from hard plastic will not brake or ware out. thanks.air-deflecter.com

mikee said...

What is the "worse-case-scenario" of having water in that dryer and through-out the system? What if my boss is saying to just drain the water daily, without replacing or repairing the dryer. Will this procedure cause a major brake failure?? Thanks for your valued input.

John Whelan said...

The air dryer MUST be serviced at least once a year. the filter will get saturated with oil and will lose it's ability to absorb. Make sure the heater (bottom of the dryer) is working as well.

Anonymous said...

A school bus driver is required to "Drain Daily" the air system on the bus. Does this mean to drain any water or drain the entire system of compressed air?