Thread: Brakes for the Impala

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Aggressive driving (or using a lot of brake when towing) generates heat. Brake pads can reach extremely high temperatures (OEM brake pads operate best in the 100°F to 650°F range while racing and high performance pads need to be at 500° F plus to become efficient – just about where stock, OEM pads begin to fade.) Some of this heat is passed onto the brake fluid through the brake calipers, which can raise the fluid temperature to well over 300°F. Fortunately, the boiling point for brake fluid is significantly higher than water. DOT 3 boiling point is 401 °F dry and 284 °F wet. DOT 4 boiling point is 446 °F dry and 311 °F wet. DOT 5 boiling point is 500 °F dry and 356 °F wet. (Wet boiling point defined as 3.7% water by volume.)

If the brake fluid is repeatedly heated past its boiling point, some of the fluid vaporizes and creates air bubbles within the brake lines. This can lead to a bad and potentially unsafe situation where the brake lines are carrying air instead of brake fluid. Air can be compressed which is why it must be bled out of the systems so that the fluid can properly move the pistons inside the brake caliper and properly operate the brakes. High temperatures also promote condensation (from vapor) and water develops in the lines (ever seen rusty brake fluid?) and this is a huge problem as shown in the above figure – wet boiling point is much lower.

Fluids don’t compress much (in liquids, there is no gap between particles, so only way you can compress fluid is by reducing the gap at a molecular level, which is effectively changing the state of that material, which involves a lot of energy – but as mentioned, brake lines do flex.

The advantage to larger lines is greater volume of fluid to dissipate the heat and keep the system well below the boiling point. With DOT 5 fluids, you’d be hard pressed to generate enough heat to boil fluid in a properly operating system – regardless of the line size.

Also, in reference to the water vapor comment:

Brake fluid is hydrophilic. Which means that it will suck water right out of the air. That's why your brake lines get rusty