Trailer brake failures

Trailer brakes don't always work as they're designed to. When they don't, overall braking forces are reduced and vehicle-stopping distances increased. The Canadian Maintenance Council noted that the loss of braking force on two of five axles (loss of trailer brakes on a five-axle rig) causes 60 mph to 0 mph stopping distances to increase from about 280 ft. to just over 500 ft. (Discussion Paper, June

Trailer brakes don't always work as they're designed to. When they don't, overall braking forces are reduced and vehicle-stopping distances increased. The Canadian Maintenance Council noted that the loss of braking force on two of five axles (loss of trailer brakes on a five-axle rig) causes “60 mph to 0 mph” stopping distances to increase from about 280 ft. to just over 500 ft. (Discussion Paper, June 2003.)

Such trailer-brake failures cause near-miss incidents, crashes, injuries and deaths. Like me, you may have thought such incidents were rare. After all, advanced dual braking systems have been standard equipment since 1992. But discussions with Zurich's Canadian Fleet Specialist team have alerted us to the “all-too-frequent” occurrence of inadequate or non-existent trailer brake function.

Working with a forensic accident analyst, Zurich determined that one recent fatal truck crash was caused by an air leak that resulted when a trailer air reservoir drain was lost. Although the tractor's air compressor was able to maintain enough pressure to keep the trailer's emergency/spring parking brakes from actuating, the leak prevented the air reservoir from supplying air to a pilot valve, resulting in no air to the service-brake system. This led to a total loss of trailer brakes.

These incidents are often blamed on driver inattention or go undetected altogether. The Maintenance Council notes that a post-crash investigation generally doesn't include a thorough analysis of system functionality unless there's compelling evidence of system failure. The expertise and equipment, such as performance-based brake testing (PBBT) devices, are often unavailable for such in-depth analysis.

Why do trailer brakes fail? There's no simple answer, but contributing conditions include the following:

  • Air reservoir failure;

  • Faulty spring brake control valves;

  • Replacing spring brake control valves improperly;

  • Restricted/pinched trailer service line (e.g., improper routing or pinching at trailer crossmembers);

  • Pressure protection valves sticking due to corrosion.



We routinely check for applied air leaks during periodic or Level I CVSA roadside inspections. We also examine pushrod travel as a way to gauge braking ability. In the situation described earlier, however, these documented failures went undetected — in spite of the fact that the vehicle had been subjected to daily routine inspection practices.

How do we fix this problem? The first step is to bring it to the attention of fleets, and make sure their drivers and technicians know what to do. Industry groups in Canada, for example, are advocating for the development of training standards and maintenance procedures that would require an increased understanding of brake system functionality and the operational differences in critical brake valves and their subcomponents.

Another important step is to move away from pushrod measurement as a surrogate for brake capability. We need to quickly deploy PBBT devices — dynamometers that can accurately measure the braking force applied to each wheel position — for routine brake system inspections. These devices compare actual versus expected braking force, as well as brake balance across all wheel positions. They are uniquely capable of targeting deficiencies in brake drums and linings, relay and control valves, and air supply lines.

Earlier this year, the Federal Motor Carrier Safety Administration (FMCSA) published out-of-service standards for the use of PBBT devices in roadside inspections. Not surprisingly, few states have deployed these devices in the field. In addition, the published standards are somewhat vague, since they specify only minimum combined vehicle braking performance criteria, rather than a more desirable wheel position performance criteria.

For a copy of the June '03 Maintenance Council Discussion Paper, “Improving Trailer Brake Safety,” call Dale Holman at Truck Watch Services, 905-877-2706.




Jim York is the manager of Zurich North America's Risk Engineering Team, based in Schaumburg, IL.

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