Poorly adjusted trailer brakes can cause long stopping distances, instability, and even MOT failures for commercial users. Whether you tow for work or leisure, understanding how your trailer’s braking system operates — and spotting early warning signs — is essential for safe, compliant towing.
This guide explains how trailer brakes work, the signs they need attention, and the maintenance habits that keep your trailer road-ready.
The Basics — How Trailer Brakes Work
Most UK trailers over 750kg MAM are legally required to have fully functioning brakes. These are almost always overrun (inertia) braking systems, designed specifically for trailers.
How overrun braking works:
- When the tow vehicle slows down, the trailer naturally moves forward.
- This forward motion pushes against the coupling head — called “overrun.”
- The overrun mechanism pulls bowden cables connected to the brake drums.
- The cables activate brake shoes inside the drums, slowing the trailer.
- Many modern systems include an auto-reverse mechanism, allowing the trailer to move backwards without the brakes locking.
Key braking components:
- Overrun unit (coupling with built-in damper)
- Bowden cables that pull the brakes
- Brake drums
- Brake shoes inside each drum
- Auto-reverse mechanism
- Handbrake lever connected to the system
Because trailers don’t have hydraulic systems like cars, mechanical alignment and correct cable tension are crucial. Even small changes in cable slack or shoe wear can significantly reduce braking power.
Signs Your Trailer Brakes Need Adjustment or Service
Brake issues often start gradually — the earlier you spot them, the cheaper they are to fix.
Common signs something’s wrong:
- Longer stopping distances
If the trailer feels like it’s pushing the tow vehicle, braking force is reduced. - Pulling or swaying when braking
Indicates uneven braking on one side — often caused by misalignment, rusted mechanisms or cable issues. - Jolting when setting off
A strong “clunk” can indicate excessive play in the overrun unit or sticking brake shoes. - Visible rust, stretched or frayed cables
Bowden cables degrade over time — damaged cables MUST be replaced. - Handbrake no longer holds the trailer firmly
If the handbrake needs more clicks than normal or won’t secure the trailer, adjustment is overdue.
If you experience any of these symptoms, it’s time to schedule a brake check through our Servicing page.
How to Check Brake Adjustment (Safely)
These checks help you spot potential issues — but always avoid DIY repairs unless you’re trained. Trailer braking systems are safety-critical.
What to look for:
- Wheel rotation test:
With the trailer raised securely, rotate each wheel by hand.- Brakes should not bind excessively
- But should offer slight, even resistance
- Brakes should not bind excessively
- Cable tension:
Bowden cables shouldn’t be overly slack or excessively tight.
Slack cables lead to weak braking; tight cables cause constant drag. - Overrun unit movement:
The coupling should slide smoothly with no sticking.
A harsh or loud clunk may indicate a worn damper. - Brake shoe condition (external clues):
Dust, rust stains or uneven wear around the drum area can indicate internal wear.
If anything looks unusual, replace worn parts through Spare Parts or book a full brake service.
How Brake Wear Affects Your MOT
While many light trailers don’t require an MOT, commercial trailers and certain categories do. And even without a formal test, you are legally responsible for keeping the trailer roadworthy.
How worn brakes can cause issues:
- Failing MOT inspections for braked commercial trailers
- Being stopped by DVSA during roadside checks
- Increased liability in the event of an accident
- Insurance complications if the trailer is deemed unroadworthy
Proper brake maintenance protects you legally as well as mechanically.
Preventive Maintenance Tips
Most brake faults can be prevented with simple care and periodic servicing.
Best practices:
- Grease moving parts (hitch, jockey wheel, linkages) to prevent stiffness
- Inspect cable runs periodically for wear, corrosion or damage
- Book regular servicing, especially if you tow frequently or carry heavy loads
- Avoid parking long-term with the handbrake engaged — brake shoes can stick inside the drums
- Store the trailer on level ground to prevent uneven brake wear
If you need replacement brake shoes, cables or couplings, visit our Spare Parts page for suitable components.
Book a Professional Inspection
If you’re unsure whether your brakes are working correctly — or if something feels “off” — it’s always safer to have them checked by a specialist. Trailer brakes are mechanical systems that require precise setup.
Our team can:
- Check brake alignment
- Replace worn shoes or cables
- Adjust overrun units
- Inspect brake drums
- Assess handbrake performance
Visit our Servicing page to book a full brake inspection, or Contact us for advice tailored to your trailer type.
Takeaways
- Trailers over 750kg MAM must have functioning brakes — they’re vital for safe towing.
- Warning signs include longer stopping distances, swaying under braking, cable wear and ineffective handbrakes.
- Regular checks and adjustments prevent costly repairs and improve safety.
- When in doubt, schedule a professional brake inspection with a trailer expert.
For brake maintenance, servicing or replacement parts, Contact the team at Three Shires Trailers — we’re here to keep every journey safe and compliant.