Why must Brake Fluid be hygroscopic?

        Today we will discuss the characteristics of a hygroscopic fluid, why brake fluid must be one, and how it works.      

What does hygroscopic mean actually?

             Most people have heard the term “hygroscopic” but may not know what it means or how it applies to them. Hygroscopic fluids are those that turn absorbent when exposed to water vapor. This is often seen on house plants; if you spray your ivy with water on a cold day, for example, the leaves will curl up as they absorb water from droplets in the air. 

Why hygroscopic fluid is important?

          Many types of stored energy devices (such as batteries) must also use hygroscopic materials because some chemical reactions produce hydrogen gas during their operation; reducing the amount of water vapor in the air will decrease the production of gas during reactions.

How hygroscopic property affects brake performance?

            Because of this, brake fluid must be hygroscopic to improve braking performance by increasing adhesion between pads and rotors. A hygroscopic material is also more apt to form a seal; when an adhesive fluid comes into contact with moisture, it will seal itself off from its surroundings. When moisture reaches brake fluid through condensation or leaks, the fluid cannot penetrate the drum or rotor surface, allowing cars to stop without skidding on wet roads even if there are no brakes present.

          The self-sealing property exhibited by hygroscopic fluids may prevent damage to vehicles that overheat while parked on steamy, sunny days. Heat can raise temperatures in the drum or rotor to 240 degrees Fahrenheit (116 degrees Celsius) which is hot enough to vaporize brake fluid. If the fluid were not hygroscopic, extreme pressure may build up and damage the drums or rotors.

            Hygroscopic fluids are especially useful in areas with high humidity; this is because even if there was no water present to absorb, moisture in the air will attach itself inside of brake lines and other devices containing hygroscopic materials. As a result, less water reaches brakes to cause corrosion or other malfunctions that would not occur otherwise.


            The automotive industry has improved brake performance by adding various compounds into hydraulic fluids to make them more hygroscopic.

Hygroscopicity is just one of many critical properties that influence the effectiveness of brake fluid; with or without self-sealing capabilities, however, all vehicle brakes require hygroscopic fluids to operate properly.

Steven Hatman
Steven Hatman

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