Brakes and Your MOT: Avoiding Common Failures

When it comes to the annual MOT test, one component consistently stands out as the primary culprit for failures: your car's braking system. Dave Harrison, a seasoned mechanic at 911 Auto Repairs in Sheffield, emphatically states, “The number one cause of MOT failures is brakes. The condition of the discs and pads, that is by far number one.” This isn't just a casual observation; it's a critical warning for all motorists. Given that brakes are fundamental to vehicle safety, their thorough inspection during the MOT is non-negotiable. Understanding what testers are looking for can be the key to ensuring your vehicle passes with flying colours, keeping you safe and legal on the road.

Why Brakes Are the Top MOT Failure

The braking system is arguably the most vital safety feature in any vehicle. Its ability to stop your car efficiently and predictably is paramount, not just for your safety but for everyone else on the road. It's no surprise, then, that MOT testers dedicate significant attention to this area. As Dave Harrison highlighted, alongside wishbones and springs, brakes represent the 'top three problems which fail MOTs'. This high failure rate stems from the complex interplay of components that must all function flawlessly, from the pedal under your foot to the pads gripping the discs.

The comprehensive nature of the brake inspection means that even minor issues can lead to a failure. Wear and tear, leaks, corrosion, or even an illuminated warning light can be enough to deem your vehicle unroadworthy. Many drivers might not notice gradual degradation in braking performance, but an MOT tester, using specialised equipment and detailed visual checks, will identify problems that could compromise safety.

The MOT Brake Inspection: What's Checked?

The MOT brake test is thorough, designed to assess both the efficiency and physical condition of the entire braking system. This typically begins with a performance check on a roller brake tester, which measures braking efficiency and balance across the axles. In some cases, particularly for certain four-wheel-drive vehicles, a road test with a portable decelerometer might be used instead. Beyond performance, a meticulous visual inspection covers numerous components:

• Under the Bonnet: Checks include the master cylinder and servo unit for leaks and correct operation. Visible metal or flexible brake pipes are scrutinised for corrosion, condition, fouling, or leaks.
• Inside the Vehicle: The footbrake pedal is inspected for insufficient reserve travel, excessive wear on the rubber or pivot, and correct servo assistance. The handbrake (parking brake) is checked for reserve travel, security of mountings, and corrosion. Anti-lock Braking System (ABS) warning lamps are checked for function and sequence of operation.
• Beneath the Vehicle: All visible flexible and metal brake pipes are inspected. Brake discs and drums are checked externally for condition and contamination. Brake back plates and caliper securing devices are assessed for condition and security. The condition of brake pads is checked if visible, and handbrake linkages/cables are examined.

Brake Pads and Discs: The Wear Factor

The most common cause of brake-related MOT failures often boils down to the condition of your brake pads and discs. These are the components directly responsible for creating the friction needed to slow and stop your vehicle.

Brake Pads

During an MOT, the thickness of your brake pads is critically assessed. While you might be advised to change your pads if they are below 3mm, the legal limit for brake pad thickness is 1.5mm. If any brake pad is found to be worn below this 1.5mm threshold, your vehicle will fail its MOT. Testers also look for pads worn down to their wear indicators, or those contaminated with oil or grease, which significantly reduces their effectiveness. Missing or incorrectly mounted pads are also immediate failures, posing a severe danger.

Brake Discs and Drums

Brake discs (or drums, on some vehicles) are also visually inspected. A brake disc or drum will fail if it is significantly and obviously worn, insecure, fractured, or otherwise likely to fail. Contamination with oil or grease, similar to pads, is also a major defect, as it compromises braking performance. While being worn below the manufacturer's recommended limits isn't an automatic failure in itself, significant visible wear indicates a problem that needs addressing.

The Often-Overlooked Brake Pedal

The brake pedal itself, seemingly a simple component, is subject to specific MOT checks. Issues here can easily lead to a failure:

• Insufficient Reserve Travel: If the footbrake pedal can be pressed so far that it touches the floor, it indicates insufficient reserve travel, which is a major defect. This means the braking system isn't engaging early enough or has too much play.
• Worn Pedal Surface: A brake pedal rubber is an anti-slip material. While it's not a defect if the design pattern of an aftermarket rubber is worn smooth, a brake pedal without a rubber (which usually has grooves or raised sections for grip) must be rejected if its grooves or raised grip sections are worn smooth. However, some vehicles are manufactured without these features, and these will not be failed.
• Servo Assistance System: The MOT tester will check for incorrect operation of the servo assistance system. This system boosts your braking power, and if it's not working properly, your pedal might feel excessively hard, or braking efficiency could be severely reduced.
• Brake Pedal Pivot: Excessive wear on the brake pedal pivot can also lead to an MOT failure, as it can affect the stability and reliable operation of the pedal.

Under the Bonnet: The Hydraulic Heart

What's happening in your engine bay is crucial for effective braking. The MOT includes checks on key components accessible under the bonnet:

• Master Cylinder and Servo Unit: These are checked for leaks with the engine running and brakes applied. The servo unit's correct operation is vital; it can be checked by depressing the pedal with the engine running and switching off the ignition – a slight increase in pressure indicates it's working. If the servo is defective or inoperative, it's a major or even dangerous defect. A leaking master cylinder is a dangerous defect.
• Brake Fluid: The brake fluid level is checked in transparent reservoirs or where an indicator is fitted; reservoir caps are not removed. The fluid should be visible, and its level must not be significantly below the minimum mark. Contaminated brake fluid is also an immediate failure, as it can compromise the entire hydraulic system. An illuminated brake fluid warning light or a defective warning device is also a defect.
• Rigid Brake Pipes: Visible metal brake pipes are checked for corrosion, condition, fouling, or leaks. Significant chafing, corrosion that reduces wall thickness, or pipes at imminent risk of failure are dangerous defects. Inadequately clipped or supported pipes are also noted.

Beneath the Vehicle: The Hidden Dangers

A significant portion of the brake inspection occurs underneath the vehicle, where many critical components are exposed to road grime and potential damage:

• Flexible Brake Hoses: These hoses connect rigid pipes to the calipers and are crucial for allowing wheel movement. They are checked for damage, chafing, twisting, stretching, and bulging under pressure. Any damage severe enough to expose reinforcement, or any leaks, constitute a dangerous defect. Porous hoses are also a major concern.
• Discs and Drums (External): Beyond the pads, the external condition of the discs and drums is checked for significant wear, insecurity, fractures, or contamination.
• Brake Back Plates and Caliper Securing Devices: These components must be secure and in good condition. An insecure brake drum back plate, for example, is a major defect.
• Brake Linkages and Cables: For mechanical systems, particularly the handbrake, the condition of linkages and cables is checked. Excessive movement indicating incorrect adjustment, or a missing/defective control, are major defects.
• Brake Compensating Valve: On some vehicles, this valve, located beneath the car, is inspected for fluid leaks.

Brake Performance: Beyond the Visual

The performance check is where the braking system’s true effectiveness is measured. Using specialised rolling road equipment, the efficiency and balance of the service brake (footbrake) and parking/emergency brake are assessed:

• Efficiency: The brakes must meet minimum efficiency requirements. A service brake efficiency less than the minimum requirement is a major defect, while less than 50% of the required value is dangerous.
• Balance: This measures how evenly the brakes apply force across an axle. Significant imbalance (e.g., braking effort from one wheel being less than 70% of the maximum from the other on the same axle, or less than 50% on a steered axle) can cause the vehicle to pull severely to one side during braking, leading to a major or dangerous failure.
• Specific Issues: Testers also look for problems like a brake grabbing severely, abnormal lag in brake operation on a wheel, or excessive fluctuation in brake effort through each wheel revolution. A significant brake effort recorded with no brake applied indicates a binding brake, which is also a major defect.

Advanced Braking Systems: ABS & EBS

Modern vehicles often feature Anti-lock Braking System (ABS) and Electronic Braking System (EBS), which are also part of the MOT test:

• ABS (Anti-lock Braking System): The ABS warning lamp is checked for correct function and whether it indicates a system malfunction. If the warning light is illuminated or defective, it's a major defect. Missing or damaged wheel speed sensors, or damaged wiring, will also result in a failure. It’s important to note that intentionally disabling or removing the ABS from a vehicle first used on or after 1st January 2010 is not permissible and will lead to an automatic failure.
• EBS (Electronic Braking System): For vehicles with electronically controlled air braking systems, the warning lamp operation is checked for similar reasons to ABS.

Common Brake MOT Failures and Their Severity

Understanding the categories of defects can help you prioritise repairs. Defects are categorised as Minor, Major, or Dangerous, with Major and Dangerous defects resulting in an MOT failure.

Frequently Asked Questions About Brake MOT Failures

Can a worn brake pedal rubber cause an MOT failure?

Yes, it can. If your brake pedal has an anti-slip rubber fitted and it's missing, loose, or worn smooth, it's a major defect. If your pedal doesn't have a rubber but has grooves or raised grip sections for anti-slip purposes, and these are worn smooth, it will also fail. However, if your vehicle was manufactured with a smooth pedal without grooves or provision for an anti-slip material, it will not be rejected.

What is the minimum brake pad thickness for an MOT?

While you might be advised to change your brake pads when they reach 3mm, the legal minimum thickness for a brake pad during an MOT test is 1.5mm. If any pad is worn below this threshold, your vehicle will fail.

Will an illuminated ABS warning light fail my MOT?

Yes, an illuminated ABS warning light indicates a system malfunction and will result in a major defect, causing your vehicle to fail its MOT. The warning light must function correctly (i.e., illuminate briefly when the ignition is switched on and then go out) and not remain lit while driving.

How is brake efficiency measured during an MOT?

Brake efficiency and balance are primarily checked using a roller brake tester. This equipment simulates road conditions and measures the braking force produced by each wheel. In specific cases, such as certain 4x4 vehicles, a road test with a portable decelerometer may be used instead.

Can contaminated brake fluid cause an MOT failure?

Yes. If the brake fluid can be clearly seen to be contaminated (e.g., with oil or debris), it is considered a major defect and will lead to an MOT failure. Contaminated fluid can severely compromise the hydraulic braking system's effectiveness.

In conclusion, the braking system is a complex and highly scrutinised part of the MOT test for good reason. From the condition of your pads and discs to the integrity of your pipes and the functionality of advanced systems like ABS, every element plays a crucial role in your safety. Regular maintenance and addressing any unusual brake noises or performance issues promptly are your best defence against an MOT failure. Don't wait until your MOT is due to check your brakes; a proactive approach ensures both compliance and, more importantly, your peace of mind on the road.

If you want to read more articles similar to Brakes and Your MOT: Avoiding Common Failures, you can visit the Brakes category.