Brake Fluid in Cars: Meaning, Uses, Function & More
What is Brake Fluid?
Brake fluid is a hydraulic fluid used in hydraulic brake and hydraulic clutch automotive applications. Its purpose is to transfer force into pressure, and to provide lubrication for the internal mechanisms in the master cylinder, wheel cylinders and calipers. Unlike oils and transmission fluid, which provide lubrication only, brake fluid has to be able to transfer force hydraulically as well (reference: https://www.romeovilletoyota.com/stop-right-there-understanding-the-importance-of-brake-fluid/).
Brake fluid is different from other automotive fluids like motor oil, transmission fluid, or coolant. While those fluids mainly provide lubrication and cooling, brake fluid’s primary role is to hydraulically transmit force applied to the brake pedal to the wheel brakes to stop the vehicle. It has a higher boiling point than those other fluids to operate under the heat and pressure generated in the braking system (reference: https://www.michiganautoandtire.com/the-benefits-of-brake-fluid-why-you-shouldnt-ignore-them/).
Types of Brake Fluid
There are three main types of brake fluid used in most vehicles today: DOT 3, DOT 4, and DOT 5.
DOT 3 and DOT 4 brake fluids are glycol-ether based fluids, while DOT 5 is a silicone-based fluid. Glycol-ether fluids like DOT 3 and DOT 4 absorb water over time, which lowers their boiling points. DOT 5 does not absorb water, so its boiling point does not change over time.
DOT 3 brake fluid has a minimum wet boiling point of 140°C and a minimum dry boiling point of 205°C. It provides good corrosion resistance and satisfactory performance under normal driving conditions. DOT 3 fluid is commonly used in older vehicles. It has largely been replaced by DOT 4 fluid in newer vehicles.
DOT 4 fluid has a higher minimum wet boiling point (155°C) and minimum dry boiling point (230°C) than DOT 3. It provides better high temperature performance for more demanding driving conditions. DOT 4 is now the most commonly used brake fluid. Most modern vehicles use DOT 4 from the factory.
DOT 5 brake fluid has a wider operating temperature range with a minimum wet boiling point of 260°C and dry boiling point of 300°C. However, it is not compatible with the rubber seals used in most brake systems designed for DOT 3 or DOT 4 fluids. DOT 5 is primarily used in systems specifically designed for its unique properties.
How Brake Fluid Works
Brake fluid works through hydraulic principles to transfer force from the brake pedal to the brake calipers, which then engages the brake pads. When the driver presses on the brake pedal, it creates hydraulic pressure in the brake lines that contains the brake fluid. This incompressible brake fluid then transfers the pressure to the brake calipers at each wheel. The calipers then use this hydraulic force to squeeze the brake pads against the surface of the brake rotors, creating friction that slows the wheels and stops the vehicle.
Brake fluid is ideally suited for hydraulic brake systems because it is a nearly incompressible liquid. This allows the pedal force exerted by the driver’s foot to be transmitted almost completely to the calipers. The hydraulic pressure gets multiplied in the process as well, allowing high braking forces with moderate pedal pressure. Brake fluid must maintain its viscosity and boiling point as it ages, otherwise vapor pockets can form under pressure which leads to brake failure. Proper fluid levels and periodic flushing is necessary to maintain the integrity of the hydraulic system.
In summary, brake fluid’s hydraulic properties allow it to efficiently transfer pressure from the brake pedal to the calipers to safely stop vehicles. Its unique characteristics make it well-suited for automotive braking systems.
Importance of Brake Fluid
Brake fluid is one of the most critical automotive safety components. It provides the hydraulic pressure needed for your brake system to slow and stop your vehicle. Without proper levels of high-quality brake fluid, you would lose braking power and the ability to control your vehicle.
When you press your brake pedal, you are activating a hydraulic system. The pedal pushes on the master cylinder, which forces brake fluid through the lines and into each wheel cylinder. Here, the fluid pressure squeezes brake pads against the rotor to provide friction and stopping power. The harder you press the pedal, the more pressure is applied at each wheel.
Brake fluid transfers the force from your foot on the brake pedal smoothly and evenly to all four wheels, providing controlled stopping power. With compromised fluid, the braking forces are not transmitted properly. Brake fluid also lubricates and protects the brake system from corrosion.
High-quality brake fluid, changed at proper intervals, ensures the safe operation of this critical system. It is the lifeblood of your brakes and vitally important for controlling your vehicle.
Checking Brake Fluid Levels
It’s important to regularly check your brake fluid levels to ensure the brake system is functioning properly. The brake fluid reservoir is usually located under the hood towards the firewall. Look for a plastic reservoir capped with a lid and check the min and max level indicators molded into the reservoir (reference this article).
The level should be between the min and max marks – if it’s below the min mark, brake fluid should be added. Having the proper brake fluid level ensures sufficient hydraulic pressure to engage the brakes and prevent brake failure. Low brake fluid could indicate a leak in the brake system which should be addressed immediately. Always check the vehicle owner’s manual for the recommended brake fluid type and proper checking procedures.
When adding brake fluid, avoid overfilling past the max mark as this can result in fluid overflow. Top up the fluid slowly and check the reservoir level frequently. Any significant drop in fluid levels should have the brake system inspected by a professional mechanic.
When to Change Brake Fluid
The general recommendation for brake fluid changes is to flush the brake system every 2-3 years. Most vehicle manufacturers recommend changing the brake fluid as part of regular maintenance every 1-2 years or 30,000 miles driven. Some vehicles may specify longer intervals of up to 5 years based on climate and driving conditions.
Signs that brake fluid needs to be changed include:
- Brake fluid appears dark or murky
- Brake pedal feels soft or spongy
- ABS light comes on indicating issues with the anti-lock brake system
- Brakes take longer to stop the vehicle
- Brake fluid level is low in the reservoir
Brake fluid absorbs moisture from the air over time which causes it to become contaminated. Contaminated brake fluid has a lower boiling point which can lead to brake failure. Changing the brake fluid regularly helps maintain the proper viscosity and keeps key brake components from corroding. It’s recommended to change brake fluid more frequently in humid climates or for vehicles that are driven aggressively where the brakes heat up often.
Sources:
https://www.bogleheads.org/forum/viewtopic.php?t=247102
https://bobistheoilguy.com/forums/threads/brake-fluid-change.334848/
Brake Fluid Flushing
Brake fluid should be flushed periodically to remove moisture and contaminants. You should flush your brake fluid according to your vehicle manufacturer’s recommended service interval, typically every 2-3 years. Here is the process for flushing brake fluid:
You’ll need a brake bleeder kit which uses pressure or vacuum to push fresh fluid through while drawing old fluid out. Popular brake bleeder kits include the VEVOR Flush Machine, Pittsburgh Automotive Bleeder, and MotorVac Brake Vac II.
Start by top off your brake fluid reservoir with fresh fluid. Attach the brake bleeder kit, opening the bleeder screw on one brake caliper. Pump fresh fluid through the system until the outgoing fluid runs clean. Repeat the process for each brake caliper, ending with the one furthest from the reservoir. Top off with fresh fluid when complete.
Flushing your own brake fluid is an advanced procedure. Consider having it done by a professional mechanic if you are unfamiliar with the brake system. Improper flushing can introduce air into the lines, leading to brake failure.
Brake Fluid Problems
Brake fluid problems typically fall into two main categories: contamination and moisture, and leaks. Let’s explore each in more detail:
Contamination and Moisture
Over time, brake fluid can become contaminated with water, dirt, and other impurities (Source 1). This leads to a reduction in the brake fluid’s boiling point, which causes it to boil easier under hard braking. Boiling fluid results in a soft, spongy brake pedal and reduced braking ability.
Moisture contamination occurs as brake fluid absorbs water from the atmosphere through microscopic pores in rubber brake hoses and seals. The water lowers the fluid’s boiling point and can cause corrosion in the brake system.
Leaks
Brake fluid leaks reduce the amount of fluid in the brake system. Symptoms of low fluid due to a leak include (Source 2):
- Spongy brake pedal
- Increased stopping distance
- Brake warning light illumination
- Leaking fluid visible on ground
Potential sources of leaks include worn brake hoses, calipers, wheel cylinders and master cylinder. Ignoring leaks can lead to catastrophic brake failure, so immediate attention is required.
Best Practices
Following manufacturer recommendations and best practices for brake fluid can ensure proper brake system operation and longevity.
Using the manufacturer recommended brake fluid type is crucial. Each vehicle’s owner’s manual will specify the correct fluid grade and specifications for that model. Using a different fluid than recommended can reduce system performance and component life.
Proper storage and handling of brake fluid is also important. Brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. This absorbed moisture can boil under high temperatures and cause vapor lock, brake fade, and other issues. Brake fluid containers should be sealed completely when not in use. Any spilled fluid should be cleaned up immediately, as it can damage paint and other surfaces. Unused portions of fluid should not be returned to the original container, as this may introduce contaminants. Brake fluid should be changed according to manufacturer specifications, typically every 2-3 years.
Adhering to best practices ensures the brake hydraulic system remains effective. Always follow OEM procedures and recommendations for brake service and maintenance.
FAQs
Brake fluid is an essential part of your vehicle’s braking system, yet it’s often misunderstood. Here are answers to some of the most frequently asked questions about brake fluid:
What does brake fluid do?
Brake fluid transfers pressure from your brake pedal to the brake calipers, which squeeze the brake pads against the rotors to slow your wheels. It also lubricates the internal components of the braking system. [1]
When should you replace brake fluid?
Most manufacturers recommend flushing and replacing brake fluid every 2-3 years. Over time, brake fluid absorbs moisture which lowers its boiling point. New fluid ensures maximum braking performance. [2]
What color is brake fluid?
Fresh DOT 3 and DOT 4 brake fluid is clear. As it ages, it turns amber, then dark brown. This color change indicates moisture absorption and the need for replacement.
Can I top off low brake fluid levels?
Yes, but first inspect your brake system thoroughly for leaks and ensure there are no underlying issues. Use brake fluid from a sealed container to avoid moisture contamination.
Is it necessary to flush the brake fluid?
Flushing the system completely removes old contaminated fluid for maximum safety and performance. Simply topping off the reservoir doesn’t eliminate moisture buildup in the lines. [3]