Brake Shim Corrosion & MOT Woes

The often-overlooked brake shim can be a source of frustration for DIY mechanics, particularly when it comes to passing the dreaded MOT test. While seemingly minor components, these small pieces of metal play a crucial role in brake performance and can succumb to the elements, leading to issues that might just cost you a pass certificate. This article delves into the world of brake shims, exploring why they corrode, the implications for your vehicle's safety and legality, and offering practical advice for dealing with these pesky components.

What Exactly Are Brake Shims?

Brake shims, also known as anti-rattle shims or vibration dampers, are typically thin, flexible metal plates that are fitted between the brake pad and the caliper piston or bracket. Their primary purpose is to absorb vibrations that occur during braking. These vibrations, if left unchecked, can manifest as annoying noises such as squealing or rattling, reducing driver comfort. By dampening these vibrations, shims contribute to a quieter and smoother braking experience. They are usually made from steel, sometimes with a rubber or composite coating on one side for enhanced damping properties.

The Culprits Behind Shim Corrosion

Like many automotive components exposed to the harsh realities of road conditions, brake shims are susceptible to corrosion. Several factors contribute to their degradation:

• Moisture: The most significant enemy of metal. Water from rain, puddles, and even road salt splashed up from the road can settle on and around the brake components. Over time, this moisture can penetrate any protective coatings and attack the base metal of the shim.
• Road Salt: During winter months, roads are often treated with salt to prevent ice formation. This salt is highly corrosive and can accelerate the rusting process on metal parts, including brake shims.
• Brake Dust: While not directly corrosive, the accumulation of brake dust, which is a mixture of metal particles from the pads and discs, can retain moisture and create a more aggressive environment for corrosion to take hold.
• Chemicals: Various cleaning agents or de-icing fluids used on vehicles can also contribute to corrosion if they are harsh or not properly rinsed off.

The corrosion process typically starts as a light surface rust but can progress to significant pitting and flaking of the metal. As Reggie's experience highlights, this corrosion can make the shims lose their integrity, becoming distorted or brittle.

Corrosion's Impact on Brake Performance and MOT

When brake shims corrode, they can cease to function effectively. The most common issue, as seen in Reggie's case, is that they can become loose or deformed. This can lead to:

• Loose Shims: Corroded shims may no longer fit snugly. They can then rattle or even fall out entirely, especially during brake maintenance. This is precisely what Reggie encountered, with the shims getting dragged out of place during caliper reassembly.
• Reduced Damping: A corroded shim might not be able to absorb vibrations as effectively, potentially leading to increased brake noise.
• Impeded Caliper Movement: In severe cases, a heavily corroded or deformed shim could potentially interfere with the smooth operation of the caliper, although this is less common than noise or looseness.

From an MOT perspective, the tester's concern about the shims is understandable. While the MOT manual doesn't explicitly list corroded shims as a direct failure item in all cases, it does cover broader aspects of brake system integrity:

• Brake System Components: The tester will check that all brake components are in good condition and properly secured. If corroded shims are loose, damaged, or missing in a way that affects the brake's function or security, it could lead to a failure. For instance, if a loose shim were to interfere with the wheel's rotation or the caliper's operation, it would certainly be a failure.
• Advisories: Even if not a direct failure, a tester might issue an advisory notice for corroded or missing shims, recommending their replacement to prevent future problems. Reggie's concern about the tester "checking his work with a fine tooth comb" and potentially failing him for "missing items" is a valid one, especially if the shims were deemed essential for the proper functioning or security of the brake assembly.

Reggie's car passing its MOT without the shims installed suggests that, in this particular instance, their absence did not constitute a safety or legal defect according to the tester. However, this can vary depending on the specific vehicle, the tester's interpretation, and the severity of the original corrosion.

Dealing with Corroded Brake Shims: A Practical Guide

If you're undertaking brake maintenance or notice signs of corrosion on your shims, here's how to approach it:

1. Inspection and Assessment

When you remove your brake calipers, take a moment to carefully inspect the shims. Look for:

• Rust: Is there surface rust, or is it pitted and flaky?
• Deformation: Are they bent, warped, or misshapen?
• Integrity: Do they feel brittle or like they might crumble?

If the shims are only lightly surface rusted and still feel solid, you might be able to clean them up. However, for anything more significant, replacement is the prudent course of action.

2. Cleaning and Refitting (Use with Caution)

If the shims appear salvageable, you could try cleaning them. A wire brush can remove surface rust. Ensure you remove all loose material. You might also consider a rust converter treatment. However, be aware that this is often a temporary fix, and the underlying corrosion may continue. If they were loose or difficult to refit due to corrosion, as Reggie found, cleaning them might not be enough to ensure they stay in place.

3. Replacement: The Recommended Solution

The most reliable solution for corroded brake shims is to replace them. This ensures that your braking system has the intended vibration-damping components in good condition.

Where to Get Replacements

You can typically purchase new brake shims from:

• Main Dealerships: For genuine Mazda parts, as Reggie intends to do, the dealership is the best source. Genuine parts are often designed to a higher standard and offer guaranteed compatibility.
• Automotive Parts Factors: Independent motor factors often stock a range of aftermarket brake components, including shims. Ensure you buy from a reputable brand.
• Online Retailers: Numerous online stores sell car parts. Again, choose a trusted supplier.

The Replacement Process

Replacing shims is usually straightforward, but requires care:

1. Remove the brake caliper.
2. Carefully prise out the old shims (if they haven't already fallen out).
3. Clean the caliper bracket and piston where the shims sit.
4. Fit the new shims. They usually clip or slide into place. Ensure they are oriented correctly as per the manufacturer's instructions or the old shims.
5. Reassemble the caliper, ensuring the pads and shims are correctly seated.
6. Important: Before reassembling, check that the caliper bracket and slider pins are clean and lubricated. This ensures the caliper can move freely, which is vital for proper braking and prevents premature wear.

4. Addressing Other Brake Component Issues

Reggie also mentioned slightly damaging a handbrake adjuster by over-winding the piston. This is a common mistake. If you wind the piston back too far, the internal mechanism can disengage or become damaged. Taking a small amount off the damaged face with a grinding wheel, as Reggie did, can sometimes rectify minor damage, but it's crucial to ensure the component still functions correctly and safely. If in doubt, replacement is always the safer option.

The condition of the brake discs is also a critical factor. Reggie noted that his front discs were "probably serviceable for another year," but the rear discs were "quite bad and did need changing." This highlights the importance of regular brake inspections. Worn discs can lead to:

• Reduced braking efficiency.
• Increased brake noise.
• Premature wear of new brake pads.
• Potential MOT failure if they are below the minimum thickness or excessively scored/cracked.

It's always wise to replace brake pads and discs in axle sets (both front or both rear) to ensure even braking performance.

Frequently Asked Questions

Q1: Can I drive my car without brake shims?

A1: While your car may pass an MOT without them, it's not generally recommended for long-term use. Shims help reduce noise and vibration, and their absence could lead to increased brake noise or accelerated wear on other brake components over time. If they fell out due to corrosion, it's a sign that other brake components may also be degrading.

Q2: My brake shims are rusty, do I need to replace them?

A2: If the rust is superficial and the shims are still solid and well-fitted, you might be able to clean them. However, if they are pitted, flaking, or loose, replacement is the safest and most effective solution to ensure proper function and prevent potential issues.

Q3: Will replacing the shims stop my brakes from squealing?

A3: Shims are designed to reduce brake noise, so replacing corroded or missing ones *can* help stop squealing. However, brake squeal can also be caused by worn pads, glazed discs, improper pad lubrication, or even the type of brake pad material used. If new shims don't solve the problem, further investigation into the brake system is necessary.

Q4: How often should brake shims be replaced?

A4: Brake shims are not typically on a fixed replacement schedule. They should be inspected during routine brake maintenance (e.g., when changing brake pads). They are usually replaced if they are found to be damaged, excessively corroded, or are lost during the process. They are often replaced as a matter of course when fitting new pads, especially if the old ones show any signs of wear or corrosion.

Q5: My tester failed me for missing brake shims. What should I do?

A5: If your car failed the MOT due to missing or defective brake shims, you will need to replace them. Fit new, correct shims for your vehicle's make and model. Once replaced, the vehicle will need to be re-tested. Ensure the new shims are properly seated to avoid any issues with the caliper or brake operation.

Conclusion

Brake shims, though small, are an integral part of a well-functioning braking system. Corrosion can render them ineffective and potentially lead to MOT advisories or failures, not to mention increased brake noise. By understanding why they corrode and knowing how to assess and replace them, you can ensure your vehicle's brakes remain safe, quiet, and compliant with regulations. Reggie's experience serves as a valuable lesson: while sometimes minor components might seem ignorable, proactive maintenance and a thorough understanding of your vehicle's parts are key to avoiding unexpected problems and ensuring peace of mind on the road.