Having an overrun brake link that is too short is a common reason for a trailer to fail its inspection. The problem with this is that the overrun brake starts to operate too late when the towing vehicle is being braked. When the overrun mechanism on the drawbar compresses, the trailer brakes should be applied almost immediately while in the green zone (A). If the trailer brakes only begin to be applied in the yellow zone (B), there is a considerable risk that the overrun brake will soon operate poorly, since the brake parts are worn. If braking does not commence until the red zone (C), the trailer will fail on inspection.
How do you check for this?
Testing the brake operation with the handbrake is fairly easy. A tip for testing when the overrun brake starts to operate is to fit a tensioning strap from the tow ball handle back to the rear of the drawbar. Now you can tension the strap gradually and see exactly when the brakes begin to be applied.
If the overrun brake link is too short, even though the trailer brakes are properly adjusted, a common reason is that the brake operating lever is incorrectly adjusted. The brake lever must not have any play, and should touch the drawbar tube for optimal brake operation. Figure 4 shows how the brake lever should touch the drawbar tube.
Figure 3
Figure 4 Brake lever
In order to alter the brake lever position, it is easiest to screw in the short brake rod in front of the turnbuckle. The reason why it often does not help to only turn the turnbuckle is that the braking action of the handbrake is then also altered.
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