Trailer overrun brake not working – causes and solutions

If your trailer’s overrun brake is not working correctly, it is a serious safety issue that should not be ignored. A faulty overrun braking system increases stopping distances, puts stress on the towing vehicle, and can make the trailer difficult to control in traffic. This guide explains the most common causes, how to recognise the symptoms, and what you can do to fix the problem.

What is an overrun brake?

An overrun brake is the self-activating braking system fitted to most braked trailers. When the towing vehicle slows down, the trailer continues to push forward. This inertia force is transferred through the coupling head into the overrun brake mechanism, which in turn pulls the brake cables and activates the wheel brakes. The system requires no electrical connection – it works entirely on mechanical force. If you want to understand the full working principle in detail, see our guide on how the overrun brake works.

Why is the overrun brake not working?

There is rarely a single cause behind a failed overrun brake. In most cases, the problem comes down to wear, corrosion, or incorrect adjustment of one or more components. The most common causes are:

Worn or damaged brake cables. The brake cables transmit the mechanical force from the overrun unit to the wheel brakes at each hub. If a cable is frayed, seized, or has snapped, the braking force never reaches the wheels. Cables are exposed to the elements year-round and are one of the first components to deteriorate – especially on older trailers or those used in wet or salty conditions.

Worn brake shoes. Even if the cable pulls correctly, the braking effect depends on the friction material pressing against the drum. When brake shoes are worn down, they can no longer generate enough friction to stop the wheels effectively. Visually inspect the lining thickness whenever you have a wheel off.

Worn or scored brake drums. Brake drums can develop grooves, cracks, or heat damage over time. A damaged drum reduces the contact area with the shoe and may cause uneven or pulsating braking. If the drum surface is rough or visibly worn, it should be replaced rather than dressed.

Faulty or weak overrun damper. The overrun brake unit contains a damper that controls how smoothly the mechanism responds to inertia. A weak or seized damper can prevent the coupling head from sliding correctly, meaning the brake is never properly activated – or it engages erratically. You will find replacement dampers and complete overrun brakes in our range.

Stiff or corroded coupling head. The coupling head must be able to slide freely for the overrun mechanism to function. Corrosion, lack of lubrication, or physical damage can cause the head to bind, preventing any braking force from being generated. Regular cleaning and greasing of the coupling is essential maintenance.

Incorrect brake adjustment. If the brakes are adjusted too loosely, the cable travel is used up before the shoes press firmly against the drum. If adjusted too tightly, the brakes may drag continuously. Both are signs that the system needs to be re-set. Adjustment is usually done via a star wheel adjuster accessible through a slot in the brake drum backplate.

Worn wheel bearings. This is an indirect cause that is easy to overlook. Worn wheel bearings allow the hub and drum to move laterally on the stub axle. This play means the drum does not sit correctly against the shoes, resulting in inconsistent or reduced braking performance. Bearing wear also produces heat that can damage the brake shoes themselves.

How do you know the overrun brake is not working?

The most common warning signs are longer stopping distances than usual, a jerky or snatching feeling when braking, or the trailer pushing the tow vehicle forward during deceleration. You may also notice that the trailer pulls to one side under braking, which typically indicates a seized cable or binding brake on one wheel only. In some cases, you may smell burning from the brakes – this can indicate that the brakes are dragging rather than releasing fully. Any of these signs should be investigated before the trailer is used again.

Can you adjust the overrun brake yourself?

Yes, in many cases the overrun brake can be adjusted without specialist tools. The wheel brakes are adjusted by turning the star adjuster inside the drum until the shoes lightly touch the drum, then backing off slightly so the wheel spins freely. The brake cable tension should also be checked – there should be a small amount of free play in the cable before it starts to take up. After any adjustment, always carry out a low-speed brake test in a safe area before driving on public roads. A good starting point is to read our guide on how wheel brakes work so you understand what you are looking for.

When should parts be replaced?

If adjustment alone does not solve the problem, it is time to inspect and replace worn components. As a rule, brake shoes should be replaced when the friction lining is worn to around 2 mm or less. Cables should be replaced if they show any sign of fraying, kinking, or corrosion – do not wait for a complete break. The overrun damper should be replaced if it no longer provides smooth resistance when the coupling head is pushed in by hand. For a full step-by-step process, see our guide on how to replace the brake cable.

Common mistakes to avoid

Many overrun brake problems are made worse – or caused – by avoidable errors. The most frequent ones are: continuing to use the trailer after the first signs of brake problems appear, over-tightening the brake cable so the brakes never fully release, neglecting to lubricate the coupling head and overrun mechanism, and failing to inspect the brake system after the trailer has been stored for a long period. Moisture and corrosion can seize cables and mechanisms during storage even if the trailer was in good condition when it was last used.

How to prevent overrun brake problems

Regular maintenance is the most effective way to prevent problems. At minimum, you should inspect the brake shoes, cables, drums, and coupling mechanism once a year or before any heavy use season. Lubricate the coupling head and overrun unit with a suitable grease, and make sure no moisture can sit in areas prone to corrosion. If the trailer is equipped with a jockey wheel, check that it retracts fully so the trailer sits level when hitched – an incorrectly positioned jockey wheel can affect the coupling geometry and put undue stress on the overrun mechanism. Also check the trailer lighting and connections at the same time to make the inspection efficient.

Is it dangerous to tow with a faulty overrun brake?

Yes. A trailer with a non-functioning overrun brake creates a serious risk on the road. In an emergency stop, the trailer will not brake proportionally with the towing vehicle, increasing the total stopping distance significantly. On downhill stretches, the trailer can push the tow vehicle and reduce steering control. In many countries it is also a legal requirement for trailers above a certain weight to have fully functioning brakes, meaning a faulty overrun system could result in a failed roadworthiness inspection or enforcement action. Always repair a known brake fault before using the trailer.

Summary

A trailer overrun brake that is not working is a mechanical fault that must be diagnosed and repaired before the trailer is used. The most common causes are worn brake cables, brake shoes, or brake drums, a faulty overrun damper, a stiff or corroded coupling head, and incorrect brake adjustment. Start by identifying which component is at fault through a visual inspection, then adjust or replace as needed. Most of these repairs can be done with basic tools at home. You will find all the parts you need – from brake shoes and cables to complete overrun brake assemblies – in the Valeryd range.