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Air Brake System Evolution and Vulnerabilities
Air brake systems have been the backbone of heavy-duty trucks and equipment since the mid-20th century, offering reliable stopping power through compressed air and spring-actuated parking brakes. Most modern trucks use dual-chamber brake cans—service and spring brake units—mounted on each axle. While diaphragm leaks and wheel seal failures are often obvious, broken internal springs can go unnoticed, posing serious safety risks.
Terminology Note
Unlike a ruptured diaphragm, which produces a hissing sound, or a leaking wheel seal, which leaves visible oil, a broken spring inside a brake can may not show any external signs. The only clue might be uneven pushrod travel between sides or a subtle change in parking brake holding power. Drivers rarely notice these symptoms, especially if the truck still stops and parks normally.
Fleet technicians have reported discovering broken springs during routine inspections, often by bumping the brake cans and listening for a metallic rattle. This method has become part of some companies’ B-service preventive maintenance routines, especially in large fleets where failure rates are statistically significant.
Best Practices for Inspection and Replacement
In Vermont, a dump truck operator replaced all brake cans before the paving season to avoid downtime. The old units were over six years old and showed signs of corrosion. In Albuquerque, a fleet technician managing over 16,000 trucks noted that broken springs were often linked to a specific manufacturer, prompting a change in spec and inspection protocol.
One technician explained that compounding doesn’t directly break springs, but it does increase stress on slack adjusters and can lead to over-adjustment. Trucks equipped with R14 valves are less prone to this issue, as the valve prevents simultaneous application of service and parking brakes.
Recommendations for Long-Term Reliability
Broken springs inside brake cans are a silent threat in air brake systems. While the truck may still function, the compromised parking brake can lead to rollaways or reduced holding power on slopes. Through routine inspection, smart replacement strategies, and driver education, fleets can catch these failures early and maintain safe, reliable braking performance.
Air brake systems have been the backbone of heavy-duty trucks and equipment since the mid-20th century, offering reliable stopping power through compressed air and spring-actuated parking brakes. Most modern trucks use dual-chamber brake cans—service and spring brake units—mounted on each axle. While diaphragm leaks and wheel seal failures are often obvious, broken internal springs can go unnoticed, posing serious safety risks.
Terminology Note
- Brake Can: A cylindrical chamber housing the diaphragm and spring used to apply braking force.
- Pushrod Travel: The linear movement of the rod extending from the brake chamber, indicating brake application.
- Compounding: A condition where both service and parking brakes are applied simultaneously, increasing stress on brake components.
- R14 Relay Valve: A valve with built-in anti-compounding features to prevent simultaneous brake application.
Unlike a ruptured diaphragm, which produces a hissing sound, or a leaking wheel seal, which leaves visible oil, a broken spring inside a brake can may not show any external signs. The only clue might be uneven pushrod travel between sides or a subtle change in parking brake holding power. Drivers rarely notice these symptoms, especially if the truck still stops and parks normally.
Fleet technicians have reported discovering broken springs during routine inspections, often by bumping the brake cans and listening for a metallic rattle. This method has become part of some companies’ B-service preventive maintenance routines, especially in large fleets where failure rates are statistically significant.
Best Practices for Inspection and Replacement
- Visual Pushrod Check: During brake application, measure pushrod travel on both sides. Uneven movement may indicate internal failure.
- Audible Spring Test: Tap the brake can with a rubber mallet and listen for loose spring fragments.
- Replace in Pairs: While not mandatory, replacing both cans on an axle ensures balanced braking and consistent parking brake force.
- Avoid Compounding: Educate drivers not to press the brake pedal while setting the parking brake. This can stress the slack adjusters and springs, especially on older trucks without anti-compounding valves.
In Vermont, a dump truck operator replaced all brake cans before the paving season to avoid downtime. The old units were over six years old and showed signs of corrosion. In Albuquerque, a fleet technician managing over 16,000 trucks noted that broken springs were often linked to a specific manufacturer, prompting a change in spec and inspection protocol.
One technician explained that compounding doesn’t directly break springs, but it does increase stress on slack adjusters and can lead to over-adjustment. Trucks equipped with R14 valves are less prone to this issue, as the valve prevents simultaneous application of service and parking brakes.
Recommendations for Long-Term Reliability
- Include brake can inspection in every undercarriage greasing session.
- Replace brake cans every 5–7 years, especially in regions with road salt or high humidity.
- Use anti-compounding relay valves on new builds or retrofits.
- Train drivers to avoid brake pedal use during parking brake engagement.
- Keep a log of brake can replacements and failures to identify patterns.
Broken springs inside brake cans are a silent threat in air brake systems. While the truck may still function, the compromised parking brake can lead to rollaways or reduced holding power on slopes. Through routine inspection, smart replacement strategies, and driver education, fleets can catch these failures early and maintain safe, reliable braking performance.
