Moisture in Brake Fluid

Basic Description

Brake fluid is 'hygroscopic,' which means it tends to absorb moisture over time. For this reason brake fluid should always be stored in an airtight container, and the brake fluid reservoir should always have a secure, tight-fitting cap.

Technical Description

Over several years, it is possible for some moisture to enter the brake system through rubber hoses and seals. A very small increase in brake fluid moisture content can have a significant effect on braking efficiency. Brake fluid becomes very hot when under pressure and when the brakes are being applied frequently (such as when driving down hilly terrain). When there is moisture in the brake fluid, this heat causes the moisture to boil, creating steam inside the brake system. Because steam can be compressed much more than a liquid, it can cause the pedal to become soft and braking efficiency can decrease significantly. When the fluid cools back down, braking efficiency returns to normal. Some manufacturers recommend that brake fluid be replaced every two years regardless of mileage to ensure low moisture content and good corrosion protection.