Brake Fluid: A Vital Guide

A car's ability to stop is arguably more critical than its ability to move. At the heart of this essential function lies brake fluid, the most vital operational fluid in any vehicle. A deficiency or issue with your brake fluid can jeopardise the safety of not only yourself but all other road users. This article will delve into everything you need to know about this indispensable component of your car's braking system.

Hydraulic Force Transmission and Boosting

The vast majority of family cars rely on hydraulic systems, excluding the parking brake, for their braking operations. This means that the force generated when you press the brake pedal is transmitted through a specialised fluid. Unlike gases, fluids possess the inherent property of distributing force equally throughout a contained system, and crucially, they cannot be compressed. This direct force transmission ensures that the moment you apply pressure to the pedal, the entire braking system reacts instantly. While pneumatic and hybrid systems, which combine hydraulic and pneumatic elements, are found in larger vehicles like trucks and buses – hence their characteristic hissing sound when stopping – conventional cars exclusively use hydraulic systems. The parking brake, typically operated by a cable, is the sole exception. The presence of compressible gases within the brake lines can have catastrophic consequences, potentially leading to a complete brake system failure. In such a scenario, a driver would find themselves pressing a pedal that yields nothing, a terrifyingly dangerous situation with potentially unavoidable collisions.

How Does Gas Enter Brake Fluid?

Gases can infiltrate the brake fluid through two primary routes. Firstly, the intense heat generated during braking can cause any condensed water within the system to boil, creating steam bubbles. Secondly, air can ingress into the braking system through leaks. While less common, this can occur in older braking systems or if the incorrect type of brake fluid is used.

The Critical Role of Brake Fluid

Brake fluid is engineered with specific properties to ensure optimal performance and safety. These critical attributes include:

• Resistance and reliability across a wide range of high and low temperatures.
• Ensuring dependable force transmission.
• Preventing flocculation (the formation of solid particles).
• Exhibiting no adverse chemical reactions with any contacting materials within the brake system.

The Main Enemy: Water

While commercially available brake fluid is designed to meet these demanding requirements, it's not without its inherent challenges. One of the most significant is its hygroscopic nature, meaning it has a natural tendency to attract and absorb water from the atmosphere. Much like a salt block drawing moisture from the air, brake fluid gradually accumulates water over time. Fortunately, this water accumulation is often visually discernible. Fresh brake fluid typically presents as clear and golden yellow. Conversely, old and contaminated brake fluid will appear green and murky. It is imperative to recognise that when brake fluid reaches this discoloured state, the point of safe usage has been significantly exceeded, and immediate action is required.

Always Changing Completely

Before undertaking a brake fluid change, it is essential to check the current fluid level. If the level is too low, it strongly indicates a leak within the hydraulic system. This leak must be located and repaired before any new brake fluid is added. Attempting to simply top up the fluid is both incorrect and dangerous. The fresh fluid would be immediately contaminated by the existing fluid and any residual moisture, and the underlying leak would remain unrepaired, inevitably leading to future brake system failure. Therefore, brake fluid should always be changed entirely. This is typically achieved by siphoning the old fluid out or draining it through the bleeding screw located on the wheel brake cylinder. Professional garages often go a step further by using pressurised air to blow out any remaining fluid residue from the entire brake system, ensuring a completely clean slate.

Don’t Forget to Vent

Introducing fresh brake fluid is not a simple pour-and-go process. The entire brake system must be properly bled to ensure that it contains only fluid and absolutely no air. The most straightforward method involves enlisting the help of another person to operate the brake pedal. Begin with the wheel brake cylinder that is furthest from the brake pedal. For vehicles with right-hand steering, as found on the British Isles, this would be the left rear wheel. Open its air vent screw, and have your helper pump the brake pedal repeatedly until brake fluid begins to flow out. Once fluid is observed, quickly tighten the air vent screw. This procedure is then repeated sequentially on all the wheels. Throughout this process, it is crucial to monitor and maintain the brake fluid level in the reservoir.

Sticking to Maintenance Schedules

Legally, brake fluid in a car must be replaced every two years. This requirement is often checked during MOT inspections, where old or degraded brake fluid may be identified. However, the MOT inspection primarily assesses the overall function and tightness of the braking system, not the specific composition of the fluid itself. Consequently, the condition of the brake fluid is a critical factor when purchasing a used car. It's possible that the vehicle has been stationary for an extended period, or the previous owner neglected the recommended maintenance intervals. To assess the water content of your brake fluid, testing strips are readily available in the automotive accessory trade. Nevertheless, it is strongly recommended to change both the engine oil and the brake fluid when acquiring a used vehicle as a matter of good practice.

Proper Disposal of Old Brake Fluid

It is imperative not to pour old brake fluid into the sewage system. Instead, it should be treated as hazardous chemical waste. You can often pour the old fluid into the container of the new fluid you have purchased and return it to the point of sale for proper disposal. Suppliers are typically equipped to handle its disposal responsibly. Alternatively, you can arrange for its collection from an online shop or deliver it free of charge to your local waste disposal facility.

Brake Fluid Cannot Be Mixed

Brake fluid should never be topped up indiscriminately. When changing or topping up the fluid, it is essential to use the same type of brake fluid that is prescribed by the vehicle manufacturer. The correct fluid specification is usually indicated by a DOT code on the packaging. In Europe, common specifications include DOT 1 through DOT 4. DOT 5 fluid is predominantly used in American vehicles. Adhering strictly to these specifications is vital. Using the incorrect type of brake fluid can lead to damage to the components of the brake system, potentially causing leaks and contamination. Both of these issues can result in dangerous driving situations due to brake failure.

Mind the Branded Quality

The braking system is arguably the most critical safety component of any car. Therefore, all its constituent parts should always be of the highest quality, and this extends unequivocally to the brake fluid. Opting for no-name products from unknown sources carries a significant risk, as they could be counterfeit or of inferior quality, posing an incalculable danger. Brakes demand first-class materials, even if they come at a slightly higher cost. Your safety, and that of others, is unequivocally worth the investment in premium quality.

Frequently Asked Questions:

Q1: How often should brake fluid be changed?
A1: According to legal requirements and general recommendations, brake fluid should be changed every two years, regardless of mileage. This is because brake fluid absorbs moisture from the atmosphere over time, which can degrade its performance and lead to corrosion.

Q2: What happens if I don't change my brake fluid?
A2: Over time, brake fluid absorbs moisture, lowering its boiling point. This can lead to brake fade, especially during heavy braking or driving in hot conditions. The absorbed water can also cause corrosion within the brake system components, potentially leading to leaks and expensive repairs.

Q3: Can I mix different types of brake fluid?
A3: No, you should never mix different types of brake fluid. For example, DOT 3, DOT 4, and DOT 5.1 are glycol-based and can generally be mixed with each other, but DOT 5 (silicone-based) is not compatible with glycol-based fluids and should never be mixed. Using the wrong type can damage seals and cause system failure.

Q4: How do I know if my brake fluid needs changing?
A4: The most common indicator is the colour of the fluid. Fresh fluid is clear and yellowish. If it has turned dark brown or black, or if it appears murky, it's time for a change. You can also use brake fluid testing strips to measure the water content.

Q5: Is it safe to just top up the brake fluid if the level is low?
A5: No, a low brake fluid level is usually an indicator of worn brake pads or a leak in the system. Simply topping up the fluid without addressing the underlying issue is dangerous and can mask a serious problem. The leak must be repaired first, and then the entire system should be flushed and refilled.

Q6: Where can I dispose of old brake fluid?
A6: Old brake fluid should be treated as hazardous waste. Do not pour it down drains or into the environment. You can typically return it to the place where you purchased new fluid, or take it to a local hazardous waste disposal facility or an auto repair shop that accepts it.

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