Here's something most technicians know but few customers understand: glycol-based brake fluid is hygroscopic, meaning it actively absorbs moisture from the atmosphere. Even in a sealed braking system, brake fluid can absorb 2-3% water over just two years through microscopic pores in rubber hoses and seals.
Why does this matter? Water contamination is catastrophic for braking performance. Pure brake fluid boils around 230-260°C, but just 3.7% water content can drop the boiling point by nearly 100°C. During hard braking—descending mountain passes or emergency stops—this moisture vaporizes, creating compressible gas bubbles in the lines. The result? Spongy pedal feel or complete brake failure, known as "vapour lock."
This is why brake fluid specifications distinguish between dry boiling point (ERBP) and wet boiling point (WERBP)—the latter assumes 3.7% water contamination. Quality formulations like DOT 4 exceed both thresholds, maintaining safety margins even as moisture inevitably accumulates.
