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Overrun Coupling Inspection & Service Guide for Trailers UK

Regular overrun coupling inspection and servicing is essential for safe, predictable braking when towing a braked trailer. A worn or poorly maintained overrun coupling can cause harsh braking, banging noises, poor stopping performance, or even loss of control.

This guide covers what to check, how to test components, and when parts must be replaced, helping keep your trailer legal, safe, and smooth on UK roads.


What Is an Overrun Coupling?

An overrun coupling allows the trailer to brake automatically when the towing vehicle slows. As the tow vehicle decelerates, the trailer compresses the coupling, activating the braking system via the drawtube and damper.

Because it’s a mechanical system, wear, corrosion and lack of lubrication can seriously affect performance.


Overrun Capacity Check (Start Here)

  • Confirm the overrun coupling capacity matches the trailer’s gross weight

  • The coupling must always be rated equal to or higher than the trailer’s maximum authorised mass (MAM)

Using an underrated coupling is unsafe and can lead to premature failure.



Mounting & Structural Fixings

Coupling Mounting Bolts

  • Check all bolts securing the coupling to the drawbar

  • Ensure correct torque and no signs of elongation or movement

Coupling Head Bolts

  • Check tightness and condition

  • Look for cracked paint or rust trails, which can indicate movement


Coupling Head & Towball Check

Coupling Head Operation

Using a 50mm test towball, check that:

  • The coupling locates fully

  • The locking mechanism engages correctly

  • Any wear or engagement indicators function properly

To meet UK standards, a 50mm towball must measure between 49.61mm and 50.00mm.

Lubrication

  • Lubricate the coupling mechanism

  • Lightly grease inside the coupling socket

⚠️ Important:If the coupling is fitted with a 50mm stabiliser coupling, DO NOT GREASE the socket, as this will reduce stabiliser effectiveness.


Eye Fittings (If Fitted)

  • Inspect for irregular or uneven wear

  • Excessive wear can indicate incorrect coupling geometry or misalignment

  • Replace worn components immediately


Drawtube Inspection

  • Move the drawtube shaft up and down

  • Check for excessive play in the supporting bearings

Acceptable end float between the drawtube and brake lever varies by manufacturer, so always refer to the specific service instructions.


Drawtube Gaiter

  • Inspect the rubber gaiter for:

    • Splits

    • Tears

    • Perishing

A damaged gaiter allows dirt and water into the drawtube, accelerating wear. Replace immediately if damaged.


Overrun Damper Test (Critical Check)

With:

  • Handbrake off

  • Wheels securely chocked

Push the coupling in by hand.

Correct Operation

  • Moves in smoothly

  • Offers steady, uniform resistance

  • Returns back out on its own

Faulty Damper Signs

  • Does not move at all

  • Slides in with no resistance

  • Does not return outward

If any of these occur, the damper must be replaced.

⚠️ Always use the correct specification damper and follow manufacturer fitting instructions exactly.


Handbrake Mechanism Check

  • Check for correct movement and engagement

  • On ratchet-type handbrakes:

    • Inspect teeth and pawl condition

    • Ensure positive locking action

Lubricate:

  • Pivot points

  • Ratchet mechanism

  • Any grease nipples underneath the assembly



Grease Nipples

  • Apply a grease gun to all nipples on top of the coupling housing

  • Use a liberal but controlled amount of grease

  • Stop once resistance increases or clean grease appears


Breakaway Cable Inspection

Check the breakaway cable for:

  • Kinks or fraying

  • Secure attachment to the burst ring

  • Correct operation of the safety clip

If replacement is required:

  • Use a genuine replacement cable

  • Ensure it passes through a hole or guide in the chassis front

This ensures a straight-line pull if the trailer becomes detached.


Reverse Cut-Out Lever

If fitted with a self-correcting reverse cut-out lever:

  • Apply and release it

  • Confirm it returns smoothly to its normal position

A sticking reverse cut-out can cause braking issues when reversing.


Final Checks & Best Practice

  • Perform overrun coupling inspections at regular service intervals

  • Always refer to the specific coupling manufacturer’s maintenance instructions

  • Address wear early — overrun issues rarely fix themselves and usually get worse


Brent’s Bottom Line

A well-maintained overrun coupling:

  • Improves braking smoothness

  • Reduces noise and shock loading

  • Extends brake and damper life

  • Keeps your trailer compliant and safe


If the coupling feels harsh, sloppy, or unpredictable — it’s time to inspect it properly, not ignore it.

Safe towing starts at the hitch.

-Brent

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