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BY: Bob Gummersall

Introduction - Compressed air is the motive force of choice by designers of heavy vehicles. Air pressure is used to power the service brakes, emergency brakes, air bags, air horns and occasionally used to fill a low tire. When air is available on a motor home or Medium Duty Tow Rig it can also be used to inflate a boat, fill bicycle or toad tires, power air tools and blow dust out of filters etc. This article is a primer on air systems describing the functions of the components and their safe use. It is interesting to know that British Columbia is the only Province or State that required training on Air Brakes and a special endorsement on driver licenses to legally operate vehicles with air brakes. All Commercial Drivers Licenses have special endorsements required to operate air brake equipped vehicles.

Air System - An air compressor is attached to the vehicle engine to generate air to pressurize two or more air tanks. As the pressure approaches about 125 psi (pounds per square inch) in both tanks, a regulator on the compressor causes it to free wheel. The air generated by the compressor passes through an air dryer that removes any moisture created when ambient air increases in temperature as it is pressurized. This air dryer also filters foreign particles from the compressed air. When the tank pressure reaches 125 psi and the regulator stops the pressurizing of air, the air dryer is signaled to expel the water that has been trapped and it makes a distinctive "pshusss" sound. As the air in the tanks is used for stopping, leveling, honking horns, etc., the pressure goes down. When it reaches about 90 psi the regulator signals the compressor to start working again and the whole cycle starts again. The two or more air tanks provide the safety backup for situations where an air leak happens in part of the system. Each tank is isolated from the other tanks with check valves that prevents air flowing to the leaking part of the system from the parts of the system that are ok. The compressor provides air to all tanks through a manifold that biases the air to the most important tank, the one that provides air to the service brakes and emergency brakes. Some system designs have one tank for half of the brakes and another for the other half. Auxiliary uses like leveling using air bags are applied to the second most important tank. In emergency conditions, like a total break of an air line, one tank is always isolated so that there is air for use in braking.

Braking — The most important use of air on heavy vehicles is for the service and emergency brakes. The brake pedal is a valve that modulates air to the brakes proportionally to its position. When the pedal valve is pressed slightly, a few psi of pressure is applied to the drum or disk brakes on both sides of each axle. As the pedal is pressed further, more pressure is applied to the brakes. When the pedal is fully depressed then all the air available in the tanks is applied to the brakes (120 psi). Most new vehicles have ABS (Anti-Lock Braking Systems) that prevent full pressure being applied to each brake so a to prevent wheel lock up during emergency stops. ABS is a subject of another article. If no ABS system is present, then full pedal movement provides enough pressure to lock up all the wheels, that’s why you see the double black marks on the highway where large vehicles have had to stop quickly. As the pedal is released, the air pressure is released to the atmosphere making the characteristic sound. The compressor then has to make up the air pressure used by this braking process. It is therefore not advisable to "pump" the brakes to slow a vehicle descending a steep mountain grade, because the compressor typically can’t keep up with the air usage. It is advisable to keep a steady slight pressure on the air brakes to keep the vehicle at a safe speed. It is better to be in the proper lower gear, going slower and using an exhaust brake so the use of the service brakes can be saved for real emergency braking actions.

The Emergency Brake is another thing. With no pressure in the system large springs apply enough pressure to the brakes to lock them solid. When getting ready to drive these special air/spring mechanisms are pressurized to make the springs release the mechanical pressure on the brakes. The emergency brake requires a minimum of 60 psi of pressure to release the springs. Thus if there is a large leak, the compressor fails, the regulator fails etc., while you are driving when the air pressure in the tanks reaches 60 psi the spring brakes are going to lock the wheels so you better be safely at the side of the road. An audible "low pressure" warning happens at 80 psi so drivers have a reasonable chance to get the rig to the side of the road.

I recommend testing the effectiveness of the emergency brake each time you start driving. To do this, put the emergency brake on, which is usually the up position, then try to make the vehicle go forward with about 1200 rpm engine speed in low gear. The vehicle should not move. Do this test where going forward a few feet will not cause damage or injury and be ready to apply the service brakes if the emergency brake does not work.

Air Suspension — There are several designs of air suspension systems using one air bag per axle side, two air bags per side, one air bag and a leaf spring per side and it goes on. All these systems use a way to control the air pressure in each air bag to maintain a level and stable ride for the vehicle. The valves that control each air bag modulate the air by measure ride height between the axle and the vehicle frame. Air usage for this control varies depending on the road the vehicle is traveling on. On roads that curve sharply and dip repeatedly, more air is used than when traveling on a level Interstate Highway. The air compressor will then cycle more often perhaps every few minutes. When on level highway the compressor should cycle very infrequently unless there is a leak in the system somewhere.

Some motor homes use air pressure to create vacuum to operate the air conditioning system. These vacuum generators use a small amount of air and make an occasional soft psss sound. If there is a leak in the vacuum system then more than normal amount of air usage will cause more frequent cycling of the air compressor.

Air used for the Air Horns is obviously at the control of the driver and even full sounding uses only a small amount of air pressure. Other occasional uses of compressed air for fill tires or using an air tool, use varying amounts of air and sometimes require the engine on the vehicle to be operating to keep the tanks pressurized. It is not advisable to use tools that require large amounts of air. If you are painting your toad, then by all means get a stand alone air compressor.

Maintenance — The compressor gets lubrication of the engine on the vehicle and most other components do not require lubrication. Check your owner’s manual for the proper lubrication technique. Air Brakes are normally automatically adjusted. The use "automatic slack adjusters" to keep the brakes properly adjusted. If your vehicle does not have automatic adjusters then you need to learn how to get under the coach and adjust the slack adjusters. If you don’t want to pay a mechanic to do this work as a normal part of his service work, then you will have to get some training. British Columbia requires each air brake driver to demonstrate his/her ability of adjust the slack adjusters. It is necessary to drain the moisture that might have collected in the air tanks by opening a small petcock in the bottom of each tank. I recommend doing this weekly unless you never find any water in the tanks and then go to monthly. If the air dryer is working correctly, there should be very little if any moisture in the tanks. There are filters in the dryer that need to be changed periodically.

Testing — At least once per week when you are operating your vehicle, you should perform this basic Air Brake Recycling Test.

Air Leakage — With the air tanks pressurized to 120 psi, then engine off, walk around the vehicle listening for any sound of air leakage. Then have your partner press the brake pedal down and hold it down while you walk around the vehicle again listing for and sound of air leakage. If you hear a hiss sound, then there is a leak and it should be fixed.

Emergency Brake Test- Start with your tanks pressurized and operate the emergency brake, try to drive forward in low gear with about 1200 rpm on the engine. The vehicle should not move and if it does the emergency brakes are faulty. Do the test so that some forward movement of the vehicle will not cause damage or injury and be ready to apply the service brakes. Release the Emergency Brake and drive forward at 15 mph and apply the Emergency Brake. A complete and immediate stop should happen within 50 feet. If it does not stop in a straight line within that distance, then the emergency brakes are faulty.

Low Pressure Alarm — Now on totally level ground or with the wheels blocked, turn the engine off and release the emergency brake. Then start using air by repeated applications of the service brakes (pump the brakes). The "low pressure alarm" should sound when the tank pressure goes from 70 psi down to 60 psi. If it doesn’t then there is a faulty pressure monitor.

Low Pressure Emergency Brake Operation — Keep pumping the service brakes until the emergency brakes are activated by the springs. This should happen at about 25 psi. If it doesn’t then there is a faulty emergency brake control valve.

Air System Recovery — Now start the engine and watch the air gauges as pressure is returned to the tanks. Hold the engine rpm at 1000. If the air suspension system is depleted it will take 5 to 10 minutes to get the tank pressure up to 60 psi. It should take less than one minute for the tanks to go from 60 psi to 90 psi. If is takes longer, then the air compressor is faulty.

Static System — Now stop the engine and watch the air gauges. With no usage or generation taking place, the pressure in the tanks should not lose more than 5 psi in 5 minutes. If they lose more than 5 psi pressure, then there is a leak that needs to be fixed.

Driving - Driving a vehicle with air brakes is very different than driving a vehicle with vacuum assisted hydraulic brakes. I recommend anyone driving a vehice with air brakes should get some training. I have found that the local school bus trainers are willing to give this training after working hours for a nominal expense.

Summary — Air Systems are used on almost every heavy vehicle on the road today because they are reliable, effective and safe. The Air System in your vehicle will serve you well when maintained and used properly. Take care of your system and learn how to use it and you will significantly reduce your risk of brake failure or break down. Keep safe and happy enjoying this great lifestyle.