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Introduction to Air Brakes
Air brakes are commonly used on heavier vehicles, their superior clamping force allows the ability to stop heavier vehicles much easier than hydraulic systems. Because air brakes don't use hydraulic brake fluid, they don't have the same concerns regarding contaminated or boiling brake fluid. They do require clean, dry air however. While an excellent detailed description of air brake systems is covered in the Air Brakes Manual link in this section, this document will give you a quick overview of how air brakes operate.
Air brakes require air to operate (I'll bet you never would have guessed that, right?) so we need to have an air supply available. Air is fed into a series of air tanks via an on-board air compressor. This compressor is direct coupled to the diesel engine. It is generally capable of pumping up to 125 PSI and will cycle on or off as needed in order to maintain air pressure in the system. The air then flows through an air dryer, which will filter out any moisture or contaminants in the air, before arriving at a series of air tanks. The minimum air tank setup is 3 tanks but many coaches will have as many as 5 tanks, depending upon how the air-ride suspension is set up.
The first tank that the air arrives at is the wet tank, sometimes referred to as the ping tank. This is where the air initially expands and most moisture condensation will occur. After this tank it will be routed to a pair of secondary air tanks. One tank will control the front brakes and the other will control the rear brakes. After this there may be other tanks that handle suspension functions. It's also important to note that you may only see 2 physical tanks on your coach. Generally these tanks are partitioned and my 2 physical tanks do have a total of 5 partitions so they are in effect, 5 air tanks.
After the air tanks there are a series of valves that we won't get into here but are covered better in the Air Brakes Manual
Air drum brakes operate just like hydraulic drum brakes in that a pair of brake shoes expands to apply braking force against the brake drums. Rather than a hydraulic piston, air brakes use a steel shaft with cams on the end. As this shaft is rotated, the cams expand and press the shoes against the drum. An arm is attached to this cam which is called the slack adjuster. When you push on the slack adjuster arm, the shaft rotates and the cams force the shoes against the drum.
In order to operate the arm, a brake diaphragm chamber is connected to each slack adjuster arm via a steel push rod. As air is applied to the diaphragm, the rod extends and operates the slack adjuster arm, which in turn applies the brakes. As the air pressure is released a spring inside the diaphragm returns the pushrod and piston so that the brakes are released. In order to keep the shoes clear of the drum when the brakes are not applied, there is a free play adjustment, similar to a clutch in a car or light truck. This is done at the slack adjuster, which has an adjustment point to give it the required slack or free play.
Air brakes require adequate air pressure in order to operate safely. However, when the vehicle is parked it needs to remain stationary, even when there is no air pressure present. On the rear axle of your coach you will find a different sort of brake diaphragm chamber. Instead of the normal "pancake" style, which is found on the front axle, there is a larger brake chamber commonly referred to as a spring brake. Spring brakes are normally locked in the applied position by spring tension. In order to move the coach you need to build up adequate air pressure to release the spring brake when you push in the yellow parking brake knob on your dash or side console. Then, whenever you apply braking force with the foot pedal a second chamber will be fed air pressure to apply the service brakes, overriding the park brake air pressure at that time. Whenever you pull the yellow park brake knob the air will be exhausted from the spring brake primary chamber and the rear axle brakes will lock up.
Spring brakes are only used on the rear axle. If they were used on the front axle the driver would lose any steering control in the event of an air system failure. With only the rear axle locked up the driver should be able to reasonably steer the vehicle as it comes to an emergency stop. For this reason it's important to not raise the rear wheels off the ground when leveling at a campsite or you will have no parking brake. You can raise the front, but not the rear.
Inspection and Safety:
It is important to read your chassis manual to ensure that you are performing the proper maintenance and pre-trip checklist procedures. Regular checking for moisture by draining the air tanks is important as is maintaining the proper service intervals on your Air Dryer. Air brakes really aren't that hard to adjust to from either a service standpoint or a driver's point of view. Understanding them is the key.
Submitted by Mark Quasius - 2/11/06
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