View Full Version : Our Coach's Air Systems, Part 1

12-15-2016, 10:18 AM
One of our more misunderstood systems is our coach’s pneumatic system. After being misguided a couple of times in the past I took a look at our coach’s air system. What I discovered is the system is not that difficult to understand.

A great starting place is to have your coach’s air drawing and parts list. Both are available through the on-line Prevost Technical library. The other information you’ll need is on the Bendix web site. You may even want to print your air system drawing in a larger size to make tracing and reading a bit simpler. I recommend having the parts manual and the drawing because some things are easier to see on the parts breakdown and that’s the only way to identify specific parts.


Now that you have all of the reference material you need, you may still have a jaw dropping expression after looking at the coach’s air system drawing. This is where anyone that has worked on or had to troubleshoot complex systems has an advantage. Their advantage is that they simply follow 2 fundamental practices. The first is to break the complex system down into logical or functional sub systems. The second practice is to then split the sub system in half to isolate the problem and to continue to split the problem area in half until you get to an area more manageable for fault isolation. Anyone can use these practices and should.

Let’s look at a Prevost’s functional subsystems. They are supply, braking, and auxiliary. Now you’re looking at the same complex air drawing from a whole new perspective. I bet your jaw has found its way back up and been replaced with a smile.

For the discussion here, let’s focus solely on Prevost installed air systems. If you didn’t know, Prevost and the converter have their own auxiliary air systems and in some cases the two systems are combined. Again, for this discussion we’re focused on Prevost’s systems.


Let’s start with where the air starts; the supply system. The heart of the supply system is the engine driven air compressor. It feeds an air dryer that removes moisture and oil. The dryer feeds the wet tank and a return line to the compressor mounted governor. The governor signals the air compressor to stop producing pressurized air and the dryer “sneezes” to unload the pressure line when the governor set air pressure has been met.

The wet tank then supplies air to the front and rear brake main tanks and the auxiliary tank. To isolate the front and rear braking systems, air entering these tanks does so through a check valve at each tank. Once air is in the main brake tanks it cannot escape back into the supply feed.

The auxiliary air system cannot have the same priority to supply air as the braking systems. To ensure the braking systems are fulfilled before the auxiliary system, the supply line is fed through a protection valve to the auxiliary tank. The protection valve opens when either side of the valve reaches a specific pressure which is somewhere between 60 and 75psi. If your coach’s air systems leak over time you may have noticed that air builds in the front and rear systems first. At the rating of the protection valve you then hear the rush of air entering the auxiliary system.

That is basically the entire supply air system. It supplies all 3 main air subsystems, rear braking, front braking, and auxiliary. Once those systems are filled the governor effectively turns off the compressor and each of the 3 systems are protected from their air returning to the supply air system.

There’s a lot to our coach's air system, so I’ll stop here in a series of articles on our air systems.

12-15-2016, 11:27 AM
Thanks Gil! Breaking it down like this will make it a lot easier to get my brain around it.


Greg Haag
12-15-2016, 04:19 PM
Thanks Gil, great information!