Air brake chambers are the primary part of the air brake system in highway and long-haul 18-wheelers. While the front and rear brake chambers of a vehicle are slightly different, they function in conjunction with a set of oversize drum-brakes, taking the place of a normal breaking system’s hydraulic assist function.
The brake chambers are the main components that move the slack adjusters back and forth. The slack adjusters are the components that open and close the brake shoes to and away from the brake drums. When the air brakes are set, the brake chamber rods pull the slack adjusters toward the brake chambers to compress the brake shoes to the drums. When the air is released, the brake chambers extract the slack adjusters to releaser the brake shoes away from the drums.
Front Brake Air Chamber
• The front brake air chambers are solid steel housings connected to an air compressor. Each air brake chamber contains a pressure bleed valve and a push rod. The push rods have a spring return on the interior and a pressure plate the spans the width and depth of the chamber. When the brake pedal is depressed, air from the compressor is pumped into the chamber. This drives the push rod out with upwards of 1,000 pounds of force. Yoked to the end of the push rod is an S-cam.
• The S-cam is a long rod that sits in the interior of the drum brakes. When it turns, it forces out the surrounding brake shoes against the interior of the wheel drums, arresting the tires. Air pressure is used because traditional hydraulic chambers would not be able to create sufficient fluid pressure to stop the tires as quickly.
Rear Spring Brake Air Chamber
• Rear spring brake air chambers perform the same function as front brake air chambers, but they also double as a parking brake, thus their design is a little more complex. These are a dual-chamber system. The push rod and air chamber as present as usual, but behind this chamber is a second one which contains a powerfully coiled braking spring on a solid plate, much like the plate the push rod ends in. It’s kept in check by a latch which connects directly to the parking break lever in the driver’s cab.
• The brake functions normally when the pedal is depressed, but if the parking brake lever is deployed, the latch snaps away from the spring and it manually drives the push rod forward with several hundred pounds worth of force. In order to disengage the parking brake, the brake pedal must be depressed, filling the first chamber with air and forcing the spring back until the latch catches hold of it again.
Measuring a Brake Chamber
1.) Park the truck or trailer on a flat surface and apply the air brakes.
2.) Place the wheel chocks in the front of the rear wheels and in the back of the rear wheels.
3.) Release the air to the air brakes. Slide under the vehicle and locate the back of the brake chamber. Place the end of the measuring tape against the back side of the brake chamber, where the threaded rod exits the chamber.
4.) Pull the measuring tape out toward the end of the threaded rod. The rod is attached to a clevis that is attached to the bottom of a slack adjuster. Pull the measuring tape out to the end of the threaded rod inside of the clevis.
5.) Lock the measuring tape and write down the measurement from the backside of the brake chamber to the end of the rod that is attached to the clevis. If comparing an old brake chamber rod to a new brake chamber rod, make sure that the old brake chamber rod is fully extended. Measure from the backside of the old brake chamber to the end of the rod. Write down the measurement. Measure the new rod to the designated measurement. Mark the measurement with a piece of chalk.