An air brake chamber is measured by locating size markings on the brake chamber or by measuring the diameter of the clamp holding the chamber together. Measuring the diameter requires a special tool, whereas locating the size markings may require the removal of dirt, corrosion or paint. Brake chamber sizes can be found in the vehicle owner's manual.
Common sizes of brake chambers include 16, 20, 24, 30 and 36. The size refers to the square inches of effective area in the chamber, therefore a size 30 brake chamber means there are 30 square inches inside. A size 30 brake chamber has an outside diameter of 8 3/32 inches, according to Fire Apparatus & Emergency Equipment Magazine. The size of a brake chamber is important because it determines the brake adjustment limit.
Air brake chambers fill with air upon pushing the brake pedal, thereby activating pushrods that apply the brakes to the wheels. Adjustments need to be made with pushrods so that they do not exceed limits. If pushrods are over-adjusted, they can become damaged. These rods have indicators to ensure proper inspection of the air brakes. These devices should be inspected daily for wear, especially with companies that use air brakes on a regular basis.