How to Test the Air System

By Jack Bradshaw - Reprinted from Winter 2007 Motorcader

We hope everyone is having some good travel experiences seeing this great country, visiting relatives or doing what you do in your Foretravel. After being part of numerous ladies Driving Schools, I was amazed at how interested the ladies were when I conducted the walk around explaining some of the mechanical things they never heard of but are so important. One thing that stood out was when I explained and demonstrated how the air system works, and what to look for should that dreaded warning bell sound. Most, if not all, never knew what is involved in how the air system controls the braking system and how important it is to know what to look for if the warning bell does ring. So, I thought it would be a good reminder to the graduates of the school and to those unable to attend the school, understand how the system works. Now all you men who read this, let your wife read it also, don't hog the information for yourself.

Here is a quick checklist on how to test the air and brake systems.

Note: You should be on level ground with the wheels chocked for some parts of this checklist.

Low air warning system test.

Start engine and build up pressure, turn engine off, turn ignition switch back on, pump brake pedal to reduce air pressure until warning light and beeper comes on. (Should come on about 60 psi.)

Emergency/parking brake test.

Push brake knob in, continue reducing air pressure. The parking brake knob should "pop out" to indicate the parking brake is activated (25-35 psi). Start engine let air pressure build up until low air pressure warning beeper stops (70-80 psi). Push parking brake knob in & attempt to move coach forward. Should move freely without any brake drag.

Air pressure build up test.

Continue to let the air pressure build; it should not take more than four minutes for the air pressure to go from discharged (5-20 psi) to between 120 and 130 psi.

Governor cut-in/cut-out test.

When air pressure gets between 120-130 psi, the governor should cut out. The dash gauge needle stops moving. When the needle stops, pump the brake pedal to reduce the air pressure to 80 psi, release the brake pedal, the compressor starts pumping air (cutting in)! watch for needle movement. The air governor causes the air compressor to cut in between (85-90 psi).

Air leak test.

Stop engine, parking brake on, and transmission in neutral. The air pressure must not drop more than 2 psi in the first minute. Release the parking brake; the air pressure must not drop more than 2 psi in this second minute. Apply service brake and hold it. The initial pressure drop must not be more than 10 psi. Continue to hold the pedal down for one full minute. The pressure should not drop by more than 3 psi.

Service brake test.

Release parking brake and drive forward slowly, apply service brake sharply while holding the steering wheel lightly to determine whether the brakes apply without pulling to either side. Drive forward slowly again and apply the service brake gradually to check for smoothness.

Now that you know how the system works normally, you will be able to quickly diagnose and respond quickly before any mishap could occur.

See ya down the road, Jack and Bobbie


© Barry Brideau 2003-2018