Drum brake, disc brake, overrun brake: what are we talking about?

Before checking anything, you need to know what type of braking system the trailer is fitted with.

The vast majority of light and medium trailers use a drum brake, actuated by a cable or mechanical linkage. It’s a reliable, low-maintenance technology, but sensitive to moisture and corrosion — particularly on trailers that are rarely used or regularly launched into water.

The disc brake delivers better performance, better suited to heavy loads and intensive use. Maintenance is the same as a car disc: check pad thickness and disc condition.

In both cases, braking is triggered by the overrun brake, also called an inertia brake. This mechanism, housed inside the coupling head, transfers braking force to the wheels when the towing vehicle decelerates. Understanding how it works is essential for correctly diagnosing braking faults — particularly during reversing, where it behaves differently. How the overrun brake works and its behaviour when reversing is worth understanding thoroughly before carrying out any adjustment.

The components to inspect on a regular basis

A full brake check covers several distinct elements, best examined in the following order.

The overrun mechanism. It must slide freely in its housing. A seized mechanism either stops transmitting braking force altogether or transmits it continuously, causing the trailer to drag. Check its travel by hand, the cleanliness of the housing, and the absence of corrosion on moving parts. On boat trailers especially, this mechanism is particularly exposed — a single launch is enough to introduce moisture into the housing if the seals are compromised.

The cables or linkage. On a cable-operated system, check the condition of the outer casing, the absence of broken strands and the overall cable tension. A slack cable reduces braking efficiency. An overly tight cable can cause permanent braking even at rest, which heats the drums and accelerates lining wear.

The linings or pads. The minimum thickness for drum brake linings is generally 2 mm. Below that, they must be replaced. On disc brakes, check pad thickness and the disc surface for deep scoring, warping or extensive corrosion — all warning signs.

The parking brake. It must hold the trailer stationary on a gentle slope. If it doesn’t, the dedicated cable is slack or the linings are too worn to provide adequate holding force.

The drums or discs. An oval or cracked drum must be replaced. A deeply scored disc or one below the manufacturer’s minimum thickness no longer does its job properly. These components are often forgotten during inspections, yet they directly condition the effectiveness of the entire braking system. If replacement is necessary, it’s also a good opportunity to ensure the trailer axle is correctly sized for the trailer’s MAM.

How to adjust drum trailer brakes

Drum brake adjustment is typically done via an adjuster wheel or nut accessible from outside the drum, sometimes through a dedicated slot. The goal is to achieve a slight friction of the linings against the drum when spinning the wheel by hand, without locking the wheel.

On a cable brake, tension is adjusted at the turnbuckle located on the linkage, between the coupling head and the wheel brakes. Lengthen or shorten the rod until the overrun mechanism has sufficient travel with no preload at rest.

A correctly adjusted brake system produces progressive, symmetrical braking on both wheels. If the trailer pulls to one side under braking, the adjustment is uneven or one lining is more worn than the other. In that case, both sides must be replaced simultaneously — replacing only one side would immediately recreate the imbalance.

What accelerates wear and how to get ahead of it

Moisture is the main enemy of trailer brakes. A trailer stored outdoors without protection, a boat trailer regularly launched, or simply a trailer that sits unused while rust takes hold — in all these cases, the mechanical braking components degrade faster than on a trailer that is used frequently and properly maintained.

Overloading is the second factor in premature wear. A trailer regularly loaded beyond its MAM puts greater demand on the brakes, heats the drums more and wears the linings far faster than expected. Braking components are not designed to compensate for chronic overloading.

Finally, a poor initial adjustment that is never corrected leads to asymmetric wear that eventually compromises the whole system. The overall lifespan of the axle depends on these same factors. The article on the average lifespan of a trailer axle provides concrete reference points for anticipating replacement timelines.

Inspection frequency: how often should you check?

A full visual inspection once a year is the sensible minimum for an occasionally used trailer. For a trailer in regular use or operating in demanding conditions, a check every six months is more appropriate.

A few situations that call for an immediate inspection, without waiting for the next scheduled check:

• After a long period of storage, particularly outdoors
• After repeated launches in salt water
• After a loaded journey on mountain roads
• If the trailer shows unusual braking behaviour: longer stopping distance, pulling to one side, friction noise when stationary

In summary

Checking trailer brakes doesn’t require specialist skills, but it does require a method. Overrun mechanism, cables, linings, drums, parking brake: each component has its own function and its own wear criteria. Neglected braking doesn’t show — but it makes itself felt, usually at the worst possible moment. An annual inspection, supplemented by a check after any intensive use, is enough in the vast majority of cases to catch problems before they become dangerous.