Air Brake Theory Revealed

Here is an excellent video on air brakes. This is the best presentation I've ever seen that will give you a detailed overview on exactly how air brakes work. The air brake mechanical and air systems work together. This video is referring to a school bus but is the same brake system found on most transport vehicles.



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The cutaways and detailed views of the air brake system explained here will help you understand the principles and terminology that you need to know. There are many things that happen in air brake systems like moisture that builds up from the heat that is generated from the compressor.

This is why draining air tanks daily is very important especially in cold weather conditions. This is a lot of information to absorb but the power of video allows you to watch it more than one time.

Mercedes MBE Diesel Engine Air Compressor Cycles Too Often

We recently had a problem with a C2 School Bus that had a air system problem. The compressor was unloading every minute, discharging the air dryer. The compressor and air dryer were both doing their job. The air system build time was OK and the air dryer [fyi: filters the air and gets rid of moisture /oil] was doing it's part.

So we focused on an air leak in the unloader circuit. Getting back to basics the air compressor is controlled by the governor which cycles the compressor between 90 psi and 125 psi. This extends compressor life allowing it to cool and take the load off until the air system pressure drops back down to 90 psi.

Also to remove the moisture and oil residue the air dryer unloads at 125 psi as well spitting out the contaminants.
So eventually we found a leak at the unloading air line to the compressor head. The engine is a Mercedes MBE 900 and the compressor is made by Mercedes. The main problem was the fitting which is a push and lock style that leaked down fast enough to allow the unloading air pressure to drain off to atmosphere and cause the compressor to kick in and go through it's cycle once again. The advantage of having an air dryer is the fact that the abnormal discharge of air every minute is very obvious.
So once the part [Thomas # 001F/23-13301-412] was replaced everybody was happy again. So if you have a constant discharge of the air dryer way before it's time it's most likely an air leak in the unloader circuit for the compressor. I hope this mechanic information has helped you...