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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, compressing the overrun device, which transfers braking force through the brake linkage and cables to the wheel brakes. If this process is interrupted anywhere along the chain, the trailer will not brake properly.

Common Causes

Common causes of poor overrun brake performance include worn, corroded, damaged 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. Worn, scored or severely corroded brake drums can also prevent the shoes from making proper contact with the drum surface. A stiff or corroded drawtube or linkage can restrict the movement of the overrun device, while a worn or defective overrun damper can cause jerky or uncontrolled brake application. 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 correctly setting the wheel brakes and the brake linkage according to the brake manufacturer’s procedure. Simply tightening the brake cables is not a substitute for correct brake adjustment. 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. A systematic inspection of all components — cables, shoes, drums, the overrun device and its drawtube — 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 and the overrun damper. Running a trailer with worn braking components can make it unsafe or unroadworthy. Always replace components with parts that are correctly specified and compatible with your exact brake model and dimensions. On some overrun brake systems, an incorrectly positioned jockey wheel can interfere with the brake linkage, so always make sure the jockey wheel is fully raised and secured in the correct travelling position. 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, lubricate the specified lubrication points according to the manufacturer’s instructions (keeping grease away from brake linings and drum surfaces), inspect the brake drums and shoes at least once a season, and verify that the overrun device moves freely through its full travel. Catching wear early means smaller, cheaper repairs — and a trailer that stops safely when it needs to.

How the overrun brake works
How wheel brakes work
Replace brake cable
Find parts for overrun brakes
Find parts for wheel brakes