Rear Brake Imbalance: A MOT Fail?

The recent changes to MOT testing have introduced a new point of failure that many drivers might not be aware of: rear brake imbalance. This seemingly minor issue can now lead to a vehicle failing its annual inspection, particularly affecting older cars equipped with drum brakes. If your car, like the fix-up Fiesta mentioned, has recently failed its MOT on this very point, you're certainly not alone. Understanding what causes rear brake imbalance and how it's assessed during the MOT is crucial for keeping your vehicle legally on the road and, more importantly, safe.

Understanding Rear Brake Imbalance

Brake imbalance occurs when one brake is performing significantly better or worse than the other on the same axle. For the rear brakes, this means that when you apply the brakes, one rear wheel might be slowing down the vehicle much more effectively than the other. During an MOT test, brake performance is measured using a roller brake tester. This machine simulates braking and records the amount of force each wheel is exerting. A significant difference between the left and right rear wheels, often expressed as a percentage, will result in a failure.

The Culprit: Drum Brakes and Wear

As highlighted by the owner of the Fiesta, a common cause of rear brake imbalance, especially on older vehicles, is the inherent nature of drum brakes. Unlike disc brakes, drum brakes rely on brake shoes that press outwards against the inside of a rotating drum. Over time and with wear, these shoes and drums can become less efficient. A particular issue with drum brakes is their self-adjusting mechanism. While designed to maintain optimal braking as the shoes wear down, this mechanism can sometimes fail to adjust correctly, leading to one side having more effective braking than the other. This is often due to dirt, corrosion, or simply wear and tear on the adjustment components.

Why Drum Brakes are Prone to Imbalance

Drum brake systems are enclosed, which can lead to the build-up of dust and debris from the brake shoes and drums. This contamination can interfere with the smooth operation of the brake shoes and the self-adjusting mechanism. Furthermore, moisture ingress can cause corrosion on the drum or shoe contact surfaces, further reducing braking efficiency. The adjustment process in drum brakes is often mechanical, involving a star wheel or similar component. If this doesn't rotate freely, the shoes won't be positioned correctly within the drum, leading to uneven braking.

Can the Handbrake Cause an Imbalance?

The handbrake, or parking brake, is a vital safety feature and is also tested during the MOT. It typically operates on the rear wheels, often through the same braking system as the foot brake, or via a separate mechanism. If the handbrake itself is not adjusted correctly, or if one of its cables is stretched or seized more than the other, it can lead to an imbalance. However, the information suggests that if the handbrake shows little or no imbalance, it points towards a healthy drum and shoe system, free from contamination. This implies that the handbrake's performance can be a good indicator of the overall health of the rear braking system. A well-functioning handbrake, even when tested for its holding power, should demonstrate a balanced application of force to both rear wheels.

The Role of the Brake Compensator

Vehicle braking systems often incorporate a brake compensator or load-sensing valve. This component is designed to modulate the braking pressure to the rear wheels based on the vehicle's load. When the vehicle is unladen, the rear brakes receive less pressure to prevent them from locking up. Conversely, when heavily loaded, they receive more pressure. The article correctly notes that the brake compensator is unlikely to cause an imbalance in the sense of one rear wheel braking significantly harder than the other due to its design. Its purpose is to ensure balanced braking relative to the load, not to create an inherent imbalance between the left and right sides when functioning correctly.

Diagnosing and Fixing Rear Brake Imbalance

If your vehicle has failed its MOT due to rear brake imbalance, the first step is to identify the cause. This often involves inspecting the rear brake components, including the drums, shoes, and any associated hardware.

Common Checks and Fixes

• Drum Inspection: Remove the drums to check for wear, scoring, or glazing on the inside surface. Also, inspect the brake shoes for wear, contamination (oil or grease), or damage.
• Self-Adjustment Mechanism: Ensure the self-adjusting mechanism is functioning correctly. This might involve cleaning and lubricating the adjuster components or, if they are worn, replacing them.
• Handbrake Cables: Check the handbrake cables for any signs of sticking, stretching, or damage. Ensure they are properly routed and lubricated.
• Brake Fluid: While less common for imbalance, ensure there are no air bubbles in the brake lines and that the brake fluid is clean and at the correct level.
• Wheel Cylinders: Inspect the wheel cylinders for leaks. A leaking cylinder can contaminate the brake shoes and drum, leading to poor performance.

As the Fiesta owner noted, adjusting drum brakes can be a bit time-consuming but is often a straightforward DIY task for those with some mechanical aptitude. It typically involves using a brake spoon or a similar tool to turn the adjuster nut until the brake shoe is lightly kissing the drum. Repeating this for both sides, ensuring an even gap, is key.

MOT Brake Test Explained

The MOT brake test is a critical part of the inspection. It measures two main aspects of braking performance:

The recent change specifically targets the brake imbalance component for the rear axle, highlighting the importance of ensuring both rear brakes work in unison. A failure in this area indicates a problem that compromises the vehicle's stopping power and stability, especially during emergency braking.

Preventative Maintenance for Your Brakes

Regular maintenance is key to avoiding MOT failures and ensuring your brakes are always in top condition.

• Regular Inspections: Have your brakes inspected at least annually, or more frequently if you notice any changes in performance (e.g., sponginess, noises, pulling to one side).
• Keep it Clean: If you have drum brakes, consider having them professionally cleaned and adjusted periodically, especially if you live in an area prone to salt or moisture.
• Address Issues Promptly: Don't ignore any warning signs from your braking system. A small problem can quickly escalate into a more significant and costly repair, or worse, a safety hazard.
• Consider Disc Brakes: For older vehicles with drum brakes, a conversion to disc brakes on the rear axle can significantly improve braking performance and reduce the likelihood of imbalance issues, though this is a more significant modification.

Frequently Asked Questions

Q1: What is considered a 'fail' for rear brake imbalance on an MOT?

A1: While specific percentages can vary slightly, generally, a difference of more than 25-30% between the braking force of the left and right rear wheels is considered a fail. Your MOT tester will have the precise figures.

Q2: My car has disc brakes on the rear, can it still fail on imbalance?

A2: Yes. While drum brakes are more prone to adjustment issues, disc brake systems can also suffer from imbalance due to worn pads, sticking calipers, or contaminated discs.

Q3: Is it safe to drive with rear brake imbalance?

A3: It is not advisable. Brake imbalance can lead to unpredictable handling during braking, potentially causing the vehicle to pull to one side or even spin. It significantly compromises your vehicle's safety.

Q4: How can I check my rear brakes myself?

A4: A basic check involves listening for unusual noises when braking, feeling for pulling to one side, and observing the handbrake's effectiveness. However, a proper diagnosis requires the use of a brake roller tester, which is part of the MOT.

Q5: If my Fiesta failed on rear brake imbalance, what's the most likely fix?

A5: Given it's a Fiesta with drum brakes, the most probable cause is the self-adjusting mechanism in the drum brakes not working correctly due to wear or contamination. Adjusting them, or replacing worn shoes and adjusters, is typically the solution.

In conclusion, the introduction of rear brake imbalance as a definitive MOT fail point underscores the importance of a well-maintained braking system. Whether you have drum or disc brakes, ensuring they are functioning evenly is paramount for safety and compliance. Regular checks and prompt attention to any anomalies will help you avoid an MOT failure and keep your vehicle performing at its best.