How a brake chamber works
A service brake chamber has a rubber diaphragm inside a metal housing. Air from the brake valve enters one side, the diaphragm moves, and the pushrod extends. The pushrod is connected to the slack adjuster with a clevis and pin. A return spring retracts the pushrod when air is released.
A spring brake chamber, often called a piggyback, adds a second section with a powerful spring. Air pressure holds that spring compressed. When the parking brake is set or air pressure is lost, the spring extends the pushrod and applies the brake.
Types and sizes
Chamber "type" refers to the effective diaphragm area in square inches, such as type 16, 20, 24 or 30. Long stroke chambers allow more pushrod travel than standard stroke chambers of the same type. 49 CFR 393.47(b) requires the service brake chambers and spring brake chambers on each end of an axle to be the same size.
Stroke readjustment limits
49 CFR 393.47(e) lists the maximum pushrod stroke for each chamber type. For example, the limit for a type 30 standard stroke clamp-type chamber is 2 inches, and long stroke chambers have higher limits. Stroke beyond the limit means the brake is out of adjustment. Appendix A to Part 396 explains that stroke is measured with the engine off and reservoir pressure of 80 to 90 psi with the brakes fully applied.
Common shop work
- Replacing leaking or damaged diaphragms or whole chambers.
- Caging spring brakes safely before removal.
- Checking for mismatched chamber sizes across an axle.
- Measuring pushrod stroke and finding the cause of overstroke.
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