The air brake chambers are what convert the air pressure force into mechanical pushrod force which engages the brake shoes on the brake system. This topic will not address rotary brake chambers because they are not used on highway rigs. Anytime that you work with air brake chambers, you had better show some respect. These pushrods can generate thousands of pounds of force. If one of your body parts gets in the way, ….too bad, ….you lose!

Front Brake Air Chamber

At the right is a typical front brake air chamber installed on the front axle. Below, is a diagram of a front brake air chamber, courtesy of GMC Truck Division. It has a single diaphragm that pushes the pushrod to the right when air enters the chamber from the air inlet. All of these brake chambers have sizing information. The larger the chamber size, the more force the chamber can generate. Chamber sizing is an important factor in designing brake systems that will help to keep the vehicle in control during panic stops. Always make sure that replacement air chambers are the correct size for that installation. Consult the manufacturer’s specifications when replacing air chambers.

Notice the second air inlet at the top, which has a plug in it. This second inlet permits hose physical arrangement options. The push rod return spring helps to expel the air and return the rod to it’s withdrawn position when the chamber air pressure is released.

Notice that the push rod yoke is threaded onto the push rod, and a locking nut locks the yoke into place. Whenever you need to adjust the air chamber rod length, just loosen the locking nut, and rotate the yoke for proper adjustment while holding the push rod from turning.

We have also included an exploded view of the front brake chamber here on the left. Front brake chambers are fairly straight forward, and understanding the front brake chamber will aid in understanding the spring brake chambers which are described next.

Rear Spring Brake Air Chamber

The rear brake air chamber is more complicated because it serves two purposes. One part of this dual chamber is called the spring brake chamber and the other part is called the service brake chamber. The service brake chamber is what activates when you step on the brake pedal. The spring brake chamber is what is charged to release the spring brakes. These diagrams below are courtesy of Neway Anchorlok International. 

Spring Brake Chamber

Spring brakes were designed so there would always be a fail-safe method of stopping an air brake vehicle if, for some reason, all air supply pressure were suddenly lost while the vehicle is in motion. In the diagram to the right, the heavy duty spring in the right-hand compartment (spring brake chamber) forces the push rod to the left which engages the slack adjuster when there is no air pressure in the spring brake chamber.

This forms the emergency brake action to stop a vehicle in motion should the air supply fail. Release of air pressure in the spring brake chamber is what takes place when you set your air parking brakes. The parking brake valve on the dash releases the air pressure in the spring brake chambers, and the powerful spring applies hundreds of pounds of brake force on the push rod, thereby rotating the slack adjuster and setting the brakes.

In the diagram to the left, the spring brake chamber has been pressurized, and the spring brake diaphragm has compressed the heavy duty spring, which allows the push rod to release the pressure on the slack adjuster arm, which releases the parking brake. When you release the parking brake, you actually pressurize the spring brake chamber. Now the service brake chamber can move the pushrod to the left when the brake pedal is pressed. 

Service Brake Chamber

The diagram on the right shows the service brake chamber when full-service braking action is applied. Notice how the left-hand chamber air on its right side has pushed the chamber diaphragm to the left which forces the push rod to the left also.

What is not shown in this diagram is that when the service brake the chamber moved the push rod to the left, the push rod moved away from the spring brake chamber diaphragm because it is not fastened to the spring brake diaphragm. The spring brake diaphragm only pushes on the push rod (much like you would push the end of a broom handle with the palm of your hand), so the push rod is free to move to the left with the service brake chamber when service brake action is required.

There can be emergency situations where one or more service brake chambers cannot be pressurized for whatever reason. When this situation arises, most spring brake chambers have a release bolt mounted on the outside of the spring chamber housing.

You can override the spring brake feature by installing this service bolt and using a wrench to draw the spring brake diaphragm to the right, which compresses the heavy duty spring and releases the spring brake force on the push rod. This is for emergencies only and all wheels should be properly blocked as this disables the parking brake. Usually, you insert the bolt and twist it 1/4 turn to lock it into place, then you tighten the nut, In this configuration, that spring brake is disabled, but the service brake can still work. This is only to be used for emergency situations because it disables the protective spring brake feature.

Last but not least, never, ever, attempt to disassemble a spring brake air chamber unless you are fully qualified. The spring forces inside the spring brake chamber can be deadly when that heavy duty spring is suddenly released by disassembly of the spring brake chamber. Remember the old joke, where opening the peanut can would release a spring which would scare the unwary? Well, the spring brake chamber has a similar surprise which could kill you or others if you don’t know what you are doing. This spring is capable of hundreds (if not thousands) of pounds of brute force when it is released. 

DD3 Brake Chambers

Many coaches use DD3 brake chambers on the drive axle for the Park Brakes. When the park brakes are applied, air pressure is routed to the rear of the chamber that applies the brakes. At the front of the chamber are rolling “locks” which wedge themselves between the shaft and the body of the chamber. If the air were to leak off of the park brake diaphragm, these locks will hold the brakes in the applied position. To release these locks, you follow the steps listed below:

  1. Start the coach and allow the air pressure to build to 120 pounds.
  2. Release the park brakes by pressing down the park brake push/pull valve in the driver’s area.
  3. Firmly apply the Service Brakes. This causes the brake chambers to apply further than the park brakes had applied, and this releases the rolling “locks” from the shafts.

Two problems that have been encountered are listed below. Also listed will be the troubleshooting procedure for fixing it.

  1. Park brakes will not release.
    • Usually having too much air pressure applying the park brakes causes this. In the air supply line for the park brakes is a pressure regulator which is set to 85 pounds. This is the amount of air pressure used to apply the park brakes, not the 120-pound system pressure. If this regulator is set too high, or the regulator has been removed, the brakes will be applied too firmly to be released by the service brake application. Setting this regulator too low will affect the stopping/holding capabilities of the parking brake. 85# is the magic number to remember.
  2. Air is leaking out of the R-14 relay valve located above the drive axle (On ABS equipped coaches, air leaks out of the modulator valve.)
    • In the DD3 brake chamber, the park brake diaphragm separates the park brake section and the service brake section. If this diaphragm develops a hole in it, air will leak into the service brake section. When the service brakes are not applied, this section is vented to the exhaust port on the relay valve (With ABS, the exhaust port on the modulator valve.) Any air entering the service brake chamber will be exhausted out immediately thus causing the illusion of a faulty relay valve (With ABS, the modulator valve.)
    • To troubleshoot this situation, remove the service brake airline from one of the DD3 brake chambers. (This is the middle hose on the chamber) If air is leaking out of the service brake port, that is the chamber with the leaking park brake diaphragm. If air is still leaking out of the relay valve and the hose in your hand, then it is the chamber on the opposite side of the axle (On coaches with ABS, air leaking out of the RH modulator valve would mean the leaking park brake diaphragm will be in the RH DD3 brake chamber, LH modulator valve would point to a leak in the LH side.)
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