n understanding of air systems is essential for anyone considering a coach equipped with air brakes. Air brakes and the other air systems are unknowns that often make people reluctant to get involved in a large coach. The problem of brake failure has about been eliminated by the addition of spring brakes. With reasonable maintenance, a properly designed air brake system will be much safer than the hydraulic brakes on a car.

There are four main areas in the system. They are the air supply, service brakes, spring brake fail safe unit, and air driven accessories. The heart of the supply system is the air compressor. It is either belt driven by the engine or gear driven off a camshaft. As it's tied to the engine, it will run anytime the engine does.

The compressor could eventually build up enough pressure to blow the system apart if it were not for the governor. It connects to the storage tank and the compressor. When the pressure in the tank gets high enough (usually 120 psi), the governor trips, cutting off the compressor. When the pressure in the storage tank gets down to a specified level, the governor flips back and the compressor can again run normally. The system cycles as you drive.

The compressed air can no longer hold as much moisture as it did, and the moisture condenses in the lines and tanks. Many coaches have an air dryer while others let it condense out in the tanks. Drain valves are installed on the bottom of each tank (including the dryer) to get rid of the water and oil to keep it from spreading through the system.

The air is stored in at least two tanks. Most of the moisture condenses out in the first tank called the primary, main, or the wet tank. From there the air goes to the secondary or dry tank. It has less moisture, but is far from dry. In more modern buses there are two dry storage tanks for the brakes and one for accessories.

A one way valve (also called a check valve) is mounted at the input to the first tank to prevent the loss of air should the valves in the compressor go out or the main feed line break. Another safety device at the tank is the over pressure valve that will open and let some air out if the governor fails and the pressure keeps going up.

A pressure gauge lets you see that the supply system is working properly. On coaches having dual brake systems (front and rear), the gauge has two needles. For additional safety, there is an automatic device that warns the driver when the pressure goes below a safe operating level. This is usually a loud buzzer and a light.

The first part of the service brake system is the foot operated main control valve. This is the brake pedal. It works as a pressure regulator, supplying air to the rest of the system. When the brakes are applied, the air goes through a quick release valve to the brake chambers. As the pressure is not as great during disengagement, the quick release valve vents air, speeding release.

A brake relay valve may be used instead of the quick release. When the pressure from the main control valve goes up, the relay takes additional air from the supply to more quickly send the pressure to the brake chambers. When the brakes are disengaged, the relay acts as a quick release.

The brake chambers convert air pressure to mechanical motion. Most brake chambers are made in two parts that look like pie pans. A heavy rubber diaphragm (called a pancake) is placed between the lips of the two pieces, and a clamp is placed around the seam. Inside one piece is a large flat plate. One side of the plate rests on the diaphragm and the other has a push rod that goes through a hole in the center of the case. When air pressure enters the other side of the case, it pushes on the diaphragm, which pushes the plate and the push rod. There is a spring between the flat plate and the case to push the plate back when the air is released.

The push rod from the brake chamber connects to a lever called a slack adjuster. The slack adjuster then connects to a rotating shaft that goes into the brake. To adjust the brakes, a screw in the slack adjuster sets its position on the shaft. When the brakes are applied, the push rod pushes on the slack adjuster, causing it to rotate the shaft and a cam (or S-cam) between the two brake shoes. The rotating cam pushes on rollers (or pads) connected to the brake shoes. The shoes then rub against the brake drum to stop the coach in the same way as a car's brakes.

In modern coaches, nylon tubing connects the parts of the system together. On older coaches they are steel or copper. The nylon is much easier to put in and will probably last longer.

The second part of the brake system is the spring brake. "Fail Safe Unit" and "Maxie" are other common names. This is a safety device mounted on the rear brakes. They are required on new buses and trucks, but antique coaches may not have them. Even if the coach has a good hand brake, it is essential that it also have a spring brake. In most coaches, the spring brake is also the parking brake. The main component of the spring brake is a big spring. When released, it applies enough pressure to the brakes to stop the coach. An air chamber compresses the spring, allowing the service brake system to operate normally. Should the air supply fail, there will be no air pressure to hold the spring back. Instead of the coach running away due to brake failure, the spring will apply the brakes.

Having air pressure on board opens many possibilities. Air pressure can do several jobs more cheaply than by other means. It also gives control to systems, such a ride height, which traditional systems could not. Most modern coaches have air suspension systems (Eagle and Dina excepted). Air suspension systems use heavy rubber bags as the support element for the coach.

Other air operated items are: air driven windshield wipers, air assisted power steering, air shutters over the radiator, air throttle, fast idle systems, engine shutdown, air shifter for manual transmissions, and electric over air shifter for Allisons.

SPECIAL CAUTIONS

These are "no fooling" situations that can kill you! The first two are generally not recognized.

1. Working under a coach is VERY DANGEROUS if your coach has air suspension. Should the air supply to the bags be lost, the coach will drop to its lowest level. This happens very quickly. Much faster than you can move out of the way. The coach's frame should be blocked under all four corners so that this cannot happen. A coach is very heavy and can push inadequate blocking into the ground. There was a case where one corner of an Eagle pushed a 20 ton hydraulic jack about six inches into an asphalt driveway. The area on the ground should be at least as large as the foot print of the tires. Care is needed to determine where the supports should go (the frame jack points) to prevent damage to the coach. Supports made of a pile of 18 inch long 2 by 10s will work. Hard wood is preferred for the one on top where the weight of the coach rests.

2. The spring brake unit contains a spring that is stored under significant compression. If the case is opened without this spring being caged (held in place), the unit will come apart with enough force to cause serious injury or death if a part should hit someone. Before working on a spring brake unit, be sure you know how it works and that you have installed the proper bolt to cage the spring, and that it has in fact caged the spring.

3. The brakes must be released while you are under the coach adjusting them. This opens the possibility of the coach rolling. More than one wheel should be blocked to ensure the coach cannot roll. After you block the wheels, start the engine and gently try to pull forward and back. If you are convinced that you have put more forward and back force on the coach than could possibly occur naturally, you should be safe. If possible, use shop air (discussed in the Note) to keep the brake system at full pressure. This eliminates the need to run the engine while adjusting the brakes.

4. The air systems operate at about 120 psi. There are various ports where air is released during normal operation. Keep your eyes and ears well away from these. A close range blast of air at this pressure can damage your eyes and break an ear drum.