Chapter 4 — Section 3: Brakes

Brakes remain the single most common reason cars over five years old fail their MOT. Section 3 combines a physical inspection of the components with a dynamic test on the roller brake tester (RBT). The visual check catches obvious problems, corroded pipes, leaking calipers, but it's the RBT that produces the figures needed to show the car can stop within the rates set out in the Road Vehicles (Construction and Use) Regulations 1986.

01Hydraulic lines and flexible hoses

The physical check starts with the pipes and hoses. Corrosion that's more than surface deep, pitting or thinning of the pipe wall, is recorded as a Major defect. A pipe that's chafed, badly supported or kinked also fails. Flexible hoses are checked for cracking, bulging under pressure, or porosity, where fluid seeps through the rubber. Testers look closely at the ferrules where hose meets metal pipe, a common point for hidden failure, and a hose that twists or stretches on full steering lock also fails.

02Brake fluid and reservoir security

The fluid reservoir must be securely mounted with the level visible. The MOT doesn't test the fluid's moisture content, but a level below the minimum mark, or a missing or ineffective cap, is a fail. A significant leak at the master cylinder or reservoir seal is Major or Dangerous depending on how much fluid is being lost, and a brake fluid warning light that stays lit means the car cannot pass.

03Anti-lock braking system monitoring

ABS is checked mainly through the dashboard warning light. It should illuminate at ignition-on and go out shortly after the engine starts or the car reaches walking pace. A light that stays on, or one that never comes on at all, which can point to tampering or a blown bulb, is a Major defect. Testers also give accessible speed sensors and wiring a visual check for damage or loose mounting, though they aren't required to strip anything down to inspect the tone wheels.

04Service brake performance and imbalance

The RBT measures braking force in Newtons at each wheel. For most Class 4 cars first used after 1 January 1968, the footbrake must reach at least 50% efficiency, calculated against the vehicle's maximum authorised mass. Imbalance matters as much as total force: if one wheel's braking force is less than 75% of its opposite number on the same axle, that's a 25% imbalance and a fail. Sharp fluctuation in braking force under steady pedal pressure, often a sign of warped discs or oval drums, is also recorded as a fault.

05Parking brake thresholds and efficiency

The handbrake has its own efficiency threshold, 16% for most modern Class 4 cars, though this can vary on older or dual-circuit setups. The tester also checks reserve travel: if the lever reaches the end of its travel without generating enough force, or the ratchet won't hold it on, that's a fail. Electronic parking brakes are tested the same way in principle, though a decelerometer may be used where the RBT isn't suitable for the electronic engagement.

• Service brake efficiency below 50% for Class 4 vehicles.
• Parking brake efficiency below 16% for Class 4 vehicles.
• Brake force imbalance of more than 25% on any steered axle.
• Noticeable judder or fluctuation pointing to distorted discs.
• An ABS warning light indicating a fault or bulb failure.

06Disc and drum condition

Discs and drums are inspected through the wheel where possible. A disc that's badly thin, cracked or heavily pitted on the friction surface fails. Surface rust on a car that's been standing is fine, but if rust has reduced the effective friction area enough to show on the RBT, it's noted. Pads are checked for thickness: less than 1.5mm of friction material is a Major defect, and metal-on-metal wear upgrades that to Dangerous.

Between the RBT figures and the visual checks on lines and hoses, Section 3 gives a reasonably complete picture of stopping ability and hydraulic reliability. It only confirms the state of the brakes on the day, though, so it's worth keeping an eye on pad wear and fluid condition between tests rather than waiting for the next one.