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Air Brake Chamber Pushrod Stroke

To be able to stop effectively in every braking situation, all components in the air brake system, including the foundation brakes, must be properly installed and maintained by qualified technicians. Air brake chamber (or actuator) pushrod stroke must be kept within the regulation limits (or readjustment limits if in reference to manual brake adjusters, which are permitted only on older vehicles) specified by U.S. and Canadian regulations.

During day-to-day driving a driver cannot “feel” how well brakes will work during an extreme braking maneuver. The most effective check of air (drum) foundation brakes is to measure the pushrod stroke to confirm it is within federal specifications. If pushrod stroke is beyond the regulation limit, the foundation brake may no longer be capable of providing full braking force, and the brake needs servicing.

Failure to properly service brake systems is the most common reason drivers (and motor carriers) are issued vehicle out-of-service (OOS) orders. When pushrod stroke exceeds the regulation limit, something is wrong with the brake and it is a violation.

The brake chamber pushrod regulation stroke limits were established to ensure there is sufficient pushrod travel to apply full force to the foundation brake under all operating conditions. The limits are based on the size of the brake chamber and whether the chamber is a standard or long stroke design. Pushrod stroke in excess of the regulation limit not only violates federal regulations but, more importantly, begins a decline in the force – eventually to zero – at the foundation brake, which will increase the distance it takes to stop the vehicle or combination. The only way to tell when you have a pushrod stroke violation is to measure the stroke while applying the service brakes at sufficient pressure.

Most heavy vehicles on the road are equipped with self-adjusting (automatic) brake adjusters (S-ABAs). Trucks and buses manufactured in the United States after Oct. 20, 1994, and in Canada after May 31, 1996, equipped with air brakes must automatically adjust for normal wear in the brake system, thereby helping to maintain proper pushrod stroke. The use of SABAs has helped significantly reduce the rate of out-of-service brake violations.

However, even with properly working S-ABAs installed, abnormal or excessive wear or broken components can result in excessive pushrod stroke, so stroke should still be measured on a regular basis. If a vehicle has manual brake adjusters (allowed on vehicles manufactured prior to the dates above) it must be measured regularly and adjusted accordingly.

By following manufacturer recommended foundation brake maintenance intervals (for lubrication, lining replacement, wear tolerances, etc.), regularly measuring the pushrod stroke, and addressing issues as they arise, crash risk can be reduced, a company’s safety rating can improve, and the chances of a violation or OOS condition are greatly reduced.

For more information, download the “Air Brake Pushrod Stroke: Why Is It So Important?” brochure.