 What are Dry & Wet Boiling Points
The higher the boiling point, the better the protection against the brake system boiling and losing effectiveness. (Remember back to your chemistry class here. When a fluid boils, air bubbles form in the liquid. In a hydraulic system, the fluid acts like a metal bar that can push against your brake pistons. However, when air bubbles form in the brake fluid, the "metal bar" becomes very flexible, because air is much more compressible than the brake fluid. The result is a spongy feeling in your brakes because there is now a cushion of air in your brake lines that you are compressing before it pushes upon the brake fluid. Ed.)
Brake fluid is hygroscopic, which means that it likes to absorb water. When brake fluid is put into the system (from an unopened container), it is dry, meaning that there is no water in the brake fluid. At this time, its boiling point is at its highest - the "dry boiling point". Over time, the brake fluid will absorb moisture from the air, which lowers its boiling point, until it reaches the point when it has absorbed all the moisture it can. At this time, its boiling point is at its lowest - the "wet boiling point". Both boiling points are important in evaluating the protection that a brake fluid gives you.
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 Commentary
As we see from the chart, the best fluid for long-term usage is Castrol SRF. Although its DBP isn't the highest, with 270 °C of WBP it beats everyone. The boiling point falls only by 40 °C (13%). No wonder that SRF is the most expensive fluid to buy - twice as expensive as i.e. Motul RBF.
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The Comparison
