Why Brake Fluid Is Hygroscopic: Explained In 2023

2007 BMW e90 335i Brake hydraulic fluid service maintenance reminder
2007 BMW e90 335i Brake hydraulic fluid service maintenance reminder from fluidmotorunion.com

An Introduction to Brake Fluid

Brake fluid is a vital component in any vehicle’s braking system. It plays a crucial role in transferring the force applied to the brake pedal to the wheels, allowing for safe and efficient braking. But have you ever wondered why brake fluid is hygroscopic?

Understanding Hygroscopicity

Hygroscopicity refers to the ability of a substance to absorb water molecules from its surroundings. Brake fluid, specifically the commonly used DOT 3, DOT 4, and DOT 5.1 types, is hygroscopic. This means that it readily absorbs moisture from the air, leading to potential issues in the braking system.

The Importance of Brake Fluid

Brake fluid operates under extreme conditions, experiencing high temperatures and pressures. It needs to maintain its properties, such as viscosity and boiling point, to ensure optimal braking performance. However, the hygroscopic nature of brake fluid can compromise its effectiveness over time.

Moisture Contamination

As brake fluid absorbs moisture, its boiling point decreases. This can lead to a phenomenon known as brake fluid boil, where the fluid can vaporize under high temperatures. Vaporized brake fluid creates gas bubbles, which are highly compressible compared to liquid, resulting in a soft or spongy brake pedal feel and reduced braking efficiency.

Corrosion and Component Damage

Moisture-contaminated brake fluid can also lead to corrosion within the braking system. Water acts as a catalyst for oxidation, causing brake components to deteriorate over time. Corroded components can affect the smooth operation of pistons, calipers, and other critical parts, compromising overall braking performance and safety.

Regular Brake Fluid Maintenance

To ensure optimal braking performance, it is crucial to regularly maintain and monitor your vehicle’s brake fluid.

Flush and Replace

Brake fluid should be flushed and replaced periodically, as recommended by the vehicle manufacturer. This process involves removing the old fluid and replacing it with fresh, uncontaminated brake fluid. Flushing helps remove moisture, contaminants, and any degraded fluid that may have compromised the system.

Quality Assurance

When replacing brake fluid, it is essential to use high-quality, recommended fluid specified by the manufacturer. Different brake fluid types have varying compositions and boiling points, designed to meet specific vehicle requirements. Using the correct fluid ensures compatibility and optimal performance.


Understanding why brake fluid is hygroscopic is crucial in maintaining a safe and efficient braking system. Regular maintenance, including flushing and replacing brake fluid, helps prevent moisture contamination, corrosion, and component damage. By prioritizing brake fluid maintenance, you can ensure your vehicle’s braking system operates at its best, providing you with a safe driving experience.