27/06/2019
The brake system is, without a doubt, one of the most vital components of any vehicle. It's the system that allows you to control your speed and bring your car to a safe stop. Yet, within this critical system, there's a fluid that often gets overlooked or simply not given enough thought: brake fluid. While you might regularly think about engine oil or coolant, brake fluid plays an equally important, albeit silent, role in your vehicle's safety. But does it actually need to be changed? The answer is a resounding yes, and understanding why is key to maintaining a reliable braking system.
The frequency with which you should change your car's brake fluid can vary significantly depending on the manufacturer and the specific model. Your vehicle's owner's manual is the definitive guide for this information, typically found within the maintenance schedule section. For example, a 2012 Nissan Leaf might recommend replacing the brake fluid every year or 15,000 miles if subjected to 'Severe Service,' and every two years or 30,000 miles under normal conditions. In contrast, a 2012 Toyota Camry's schedule might not explicitly call for a replacement within its listed 12-year or 120,000-mile lifespan, though it will likely stipulate periodic inspections. Most vehicles fall somewhere between these two examples. If you don't have your owner's manual handy, a quick online search for "(year/make/model) owner's manual" or "(year/make/model) maintenance schedule" can often provide the necessary details. It's always best to reference the manufacturer's official website or a reputable source that displays your owner's manual directly, rather than relying on third-party sites that may offer generalised advice.
Why Does Brake Fluid Need Changing? The Science Behind It
The fundamental principle behind your braking system relies on a crucial property of liquids: they are noncompressible. This means that when you press the brake pedal, the force is efficiently transferred through the small-diameter lines (tubes) to the brake components at each wheel. This hydraulic action is what enables your car to slow down. Conversely, gases, such as air, are compressible. If any air enters your brake fluid lines, it can lead to a "mushy" or spongy brake pedal feel. This compression means that not all of the force you apply to the pedal is transferred to the brakes, which can be particularly dangerous during downhill braking or emergency stops.
So, how does air, or more importantly, a gas that can lead to brake failure, get into your brake fluid lines? The primary culprit is water. While modern brake fluids have high boiling points, typically around 440 degrees Fahrenheit, water boils at a much lower 212 degrees Fahrenheit. Brake fluid is hygroscopic, a term that means it readily absorbs moisture from the surrounding environment. Your car's brake fluid reservoir, located under the bonnet, is usually only half-filled with fluid, leaving the other half exposed to air. The same applies to any opened container of brake fluid that you might use to top up the reservoir. Air, no matter how dry it seems, always contains some level of moisture, which is more prevalent in humid climates. Therefore, any time the brake fluid reservoir or an opened fluid container is exposed to air, water can be absorbed.
This absorption of water has a significant consequence: it lowers the boiling point of the brake fluid. When you apply the brakes, especially during prolonged periods of braking like going downhill or during a spirited drive, the brake system generates a considerable amount of heat. If the brake fluid has absorbed too much water, this heat can cause the fluid to boil and turn into vapour. As we've established, vapour is compressible. This vapour formation within the brake lines can lead to a complete loss of braking power – a terrifying prospect.
Furthermore, the presence of water in the brake fluid can accelerate the rusting process within the metal brake lines and components. This corrosion can lead to leaks in the system, further compromising its integrity and potentially causing brake failure. Even if there isn't a leak, the internal corrosion can cause debris to enter the brake fluid, leading to blockages and reduced performance.
When to Change Your Brake Fluid: Intervals and Indicators
The maintenance schedule in your owner's manual will provide specific intervals for brake fluid replacement. These intervals are typically based on both time and mileage. It's crucial to adhere to the whichever comes first rule. For instance, if your schedule states replacement every two years or 30,000 miles, and you reach the two-year mark before hitting 30,000 miles, it's time for a fluid change. Conversely, if you reach 30,000 miles within the first year, you should also have the fluid replaced.
Sometimes, a maintenance schedule might only call for an inspection of the brake fluid. This inspection often goes beyond just checking the fluid level. A mechanic may perform a test to determine the percentage of water content in the fluid. If the water content is very low, generally below 3%, the fluid might not require immediate replacement. However, even if the water content is acceptable, the fluid's condition can still be a factor. Over time, brake fluid can become contaminated with dirt and debris from the braking system. Visually, new brake fluid is typically a light brown or almost clear colour. If the fluid appears dark, cloudy, or contains visible particles, it warrants a change regardless of its water content.
DIY vs. Professional Brake Fluid Replacement
Checking the water content in your brake fluid can be done with relatively inexpensive testers, often available for less than £10. However, it's important to note that these DIY testers may not always carry official certification of accuracy. For a definitive assessment and professional service, it's always best to consult a qualified mechanic. They have the specialised equipment and expertise to accurately test the fluid and perform a complete brake fluid flush.
A common practice is to have the brake fluid flushed and replaced when you undertake other brake work, such as replacing brake pads or rotors. This is often a good opportunity to ensure your brake fluid is in optimal condition, especially if you do a significant amount of highway driving where brakes are used less frequently, meaning the fluid has more time to absorb moisture between periods of high heat generation.
Signs Your Brake Fluid Needs Attention
While adhering to your manufacturer's recommended schedule is the best approach, there are also some signs that your brake fluid might need attention sooner rather than later:
- Spongy or Soft Brake Pedal: This is a classic symptom of air or moisture in the brake lines, indicating the fluid is compromised.
- Brake Warning Light: While this can indicate various issues, including low fluid levels, it can also be triggered by fluid contamination or a failing master cylinder.
- Dark or Discoloured Fluid: As mentioned, a change in the fluid's colour from its original light shade to a dark brown or black suggests contamination and degradation.
- Low Brake Fluid Level: While not directly a sign of needing a change, a consistently low fluid level could indicate a leak in the system, which needs immediate attention.
Brake Fluid Types: A Quick Comparison
It's important to use the correct type of brake fluid for your vehicle, as specified by the manufacturer. The most common types are DOT 3, DOT 4, and DOT 5.1, all of which are glycol-based and hygroscopic. DOT 5 is silicone-based and not hygroscopic, but it is not compatible with most braking systems designed for glycol-based fluids. Always check your owner's manual for the correct specification.
| Brake Fluid Type | Dry Boiling Point (°C / °F) | Wet Boiling Point (°C / °F) | Compatibility |
|---|---|---|---|
| DOT 3 | 205 / 401 | 140 / 284 | Glycol-based, hygroscopic |
| DOT 4 | 230 / 446 | 155 / 311 | Glycol-based, hygroscopic (higher performance than DOT 3) |
| DOT 5.1 | 260 / 499 | 187 / 369 | Glycol-based, hygroscopic (highest performance) |
| DOT 5 | 260 / 499 | 180 / 356 | Silicone-based, non-hygroscopic (not compatible with DOT 3, 4, 5.1 systems) |
Frequently Asked Questions
Q1: How often should I check my brake fluid?
A1: It's a good practice to check your brake fluid level and appearance at least once a month, or whenever you check your engine oil and other vital fluids.
Q2: Can I mix different types of brake fluid?
A2: You should never mix DOT 5 silicone-based fluid with glycol-based DOT 3, DOT 4, or DOT 5.1 fluids. While DOT 3, DOT 4, and DOT 5.1 are generally compatible, it's always best to stick to the type recommended by your vehicle manufacturer.
Q3: What happens if I don't change my brake fluid?
A3: If you don't change your brake fluid, it can absorb moisture, lower its boiling point, and lead to brake fade or complete brake failure, especially under heavy braking. It can also cause corrosion within the brake system.
Q4: Is brake fluid dangerous?
A4: Brake fluid can be irritating to the skin and eyes. It can also damage the paintwork on your car. Always handle it with care, wear protective gloves and eyewear, and clean up any spills immediately.
In conclusion, while brake fluid might seem like a minor component, its condition is paramount to your safety. By understanding its hygroscopic nature, the impact of moisture, and by adhering to your vehicle's maintenance schedule, you can ensure your braking system remains reliable and responsive. Don't overlook this critical fluid; it's your car's silent guardian, and it deserves your attention.
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