Trailer overrun brake not working – causes and solutions
When the overrun brake on your trailer stops working correctly, it can seriously affect road safety. The overrun brake is a mechanical system that automatically applies the trailer’s brakes when the towing vehicle decelerates. As the vehicle slows down, the trailer pushes forward against the coupling head, activating the overrun mechanism which transfers braking force through the brake cables to the wheel brakes. If this process is interrupted anywhere along the chain, the trailer will not brake properly.
Common Causes
The most frequent cause of overrun brake failure is worn or seized brake cables. If a cable is stretched, frayed, or corroded, the braking force cannot be transferred reliably to the wheel brakes. Worn brake shoes are another common issue — when the friction material is depleted, braking efficiency drops even if the overrun mechanism itself is functioning. Corroded or damaged brake drums can also prevent the shoes from making proper contact with the drum surface. In addition, a faulty overrun damper inside the overrun brake unit, or a stiff and corroded coupling head, can prevent the mechanism from activating correctly. General stiffness due to rust or lack of lubrication throughout the linkage is also a common contributing factor.
Incorrect Adjustment
Incorrect brake adjustment is one of the most overlooked causes of poor overrun braking. If the brakes have drifted out of specification or have never been set correctly, the overrun travel may not be sufficient to generate meaningful braking force at the wheel. Adjustment involves setting the correct cable tension and ensuring the brake shoes are positioned within the proper range relative to the drum. This should be checked regularly, especially after replacing any brake components.
How to Identify the Problem
The most obvious symptoms of a faulty overrun brake are longer stopping distances, a jerky or uneven braking feel, or the trailer pushing forward during deceleration. In some cases, the brakes may not engage at all during gradual slowing. Worn wheel bearings can also cause irregular drum rotation that affects braking consistency. A systematic inspection of all components — cables, shoes, drums, the overrun unit and coupling head — will usually identify the root cause.
When to Replace Parts
If inspection reveals that any component is worn, corroded, or damaged beyond its service limit, it should be replaced rather than adjusted. This applies to brake cables, brake shoes, brake drums, the overrun damper, and the coupling head. Running a trailer with worn braking components is a safety risk and is likely to result in a roadworthiness test failure. Always replace components with parts that match the original manufacturer specification for your brake model. It is also good practice to check that the jockey wheel is in good condition, as a stiff or damaged jockey wheel can affect how the coupling head moves and indirectly affect brake activation. Keep your trailer lighting in working order as part of general trailer maintenance.
Prevention and Maintenance
Regular inspection is the most effective way to prevent overrun brake problems. Check the brake cables for fraying or corrosion, clean and lubricate all moving parts of the overrun mechanism, inspect the brake drums and shoes at least once a season, and verify that the coupling head moves freely through its full overrun travel. Catching wear early means smaller, cheaper repairs — and a trailer that stops safely when it needs to.