What is Brake Fluid? Signs That It Needs to Be Replaced
When it comes to a car’s braking system, many people may only think of brake pads or rotors. However, brake fluid is also a crucial component that should not be overlooked! It acts as the medium that transmits pressure from the brake pedal to the wheels. If brake fluid deteriorates, it can increase stopping distances, reduce braking efficiency, and potentially cause dangerous situations. Understanding what brake fluid is, its types, how it deteriorates, and when it should be replaced is essential for every driver to ensure safe and confident driving.
Table of Contents
Brake Fluid: What It Is
Brake fluid is a hydraulic fluid that serves as the transmission medium in a car’s braking system. When the driver presses the brake pedal, the pressure from the master cylinder is transmitted through the brake fluid to the brake pistons, which press the brake pads against the rotors to stop the vehicle.
Brake fluid has a high moisture absorption property, especially in hot and humid climates like Thailand. Over time, brake fluid degrades and darkens in color (turning dark brown or black), which reduces braking efficiency, increases stopping distances, and may cause noise or damage to the brake rotors.
It is recommended to replace brake fluid every 1–2 years, or approximately every 40,000 kilometers, and never leave it for more than 3 years.
Types of Brake Fluid
Commonly used brake fluids are certified by SAE (Society of Automotive Engineers), DOT (U.S. Department of Transportation), and ISO (International Organization for Standardization). They are mainly divided into three types
1. DOT 3
- Dry boiling point: 205°C
- Wet boiling point: 140°C
- Glycol-based
- Suitable for standard everyday vehicles
2. DOT 4
- Dry boiling point: 230°C
- Wet boiling point: 155°C
- Glycol-based
- Suitable for high-speed driving or heavier usage
3. DOT 5
- Dry boiling point: 260°C
- Wet boiling point: 180°C
- Silicone-based
- Pros: Does not absorb moisture, so it degrades more slowly
- Caution: Do not mix with DOT 3 or DOT 4; not suitable for ABS braking systems
Factors That Cause Brake Fluid to Deteriorate
1. Heat
Frequent braking, such as on steep roads or sudden stops, can cause the brake fluid to absorb high heat, sometimes reaching 350–400°C. If the fluid boils and turns into vapor, hydraulic pressure is lost, causing brake failure and preventing the car from stopping.
2. Moisture
Brake fluid absorbs moisture from the air, especially in humid climates like Thailand. This lowers the boiling point of the fluid and corrodes metal components, leading to rust and reduced braking efficiency.
Signs That Brake Fluid Needs to Be Replaced
- The brake fluid color turns dark brown or black
- Longer stopping distances; more pedal pressure is required
- Noises when pressing the brake pedal
- A soft brake pedal or an unstable braking feel
How to Properly Maintain and Replace Brake Fluid
- Replace every 1–2 years or every 40,000 km
- Do not leave it for more than 3 years, as degraded fluid reduces braking efficiency
- Never mix different types of brake fluid (e.g., DOT 3/4 with DOT 5). If changing types, completely drain the old fluid first
- Regularly check the brake fluid level in the reservoir under the hood
Conclusion
Brake fluid is a vital part of the braking system, transmitting pressure from the brake pedal to the wheels. Over time, it deteriorates due to heat and moisture, reducing braking performance and potentially causing danger. Drivers should regularly check and replace brake fluid as recommended and use certified, suitable fluid for their vehicles to ensure safe, confident, and efficient driving on every journey.
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