Brake Fluid

The hydraulic brake system is the single most neglected part of your vehicle. The master cylinder, brake lines, and calipers are rarely ever inspected unless there’s a problem. And the weakest link in this critical component is arguably the brake fluid.

The primary function of brake fluid is to transmit force applied to the brake pedal to the brake pads. It is designed to withstand high boiling points and repel moisture absorption, while simultaneously inhibiting corrosion. But if the brake fluid becomes contaminated, it can compromise your automobile’s hydraulic system with potentially disastrous results. Even a small amount of water can reduce braking force – water boils at much lower temperatures than brake fluid – which is why we recommend that you flush the brake system in your car, truck, SUV or minivan at least every 24 months.

Brake Fluid Fundamentals

There have been a number of technological advances in brake fluid development since the 1920s, when auto manufacturers changed from mechanical brakes to hydraulic brakes. As our cars, trucks, SUVs and minivans have become faster and heavier, manufacturers have had to keep pace with the extra demands required of hydraulic brake systems. Brake fluid is an integral part of this system and must work in extreme cold temperatures without thickening or freezing, as well as in extreme heat without boiling.

As it relates to heat, brake fluids are engineered to resist boiling for two reasons. First, boiling creates vapor and air bubbles, which are not compressible. This means that pressing on the brakes will not exert pressure on the brake pads. The second reason is that if allowed to boil, the physical and chemical properties of brake fluid changes and it will boil at lower temperatures going forward. This is why moisture contamination is so detrimental, especially with glycol-ether based brake fluids which are hygroscopic (meaning they absorb water from the atmosphere).

Moisture can access your vehicle’s hydraulic system and contaminate brake fluid in a number of ways. Microscopic water molecules can enter through the vent in the master cylinder, pores in the brake hose, seals in wheel cylinders and calipers, and simply through condensation in the air space above the fluid. Water contamination from any source will lower the boiling point, increase viscosity at low ambient temperatures, and cause corrosion of brake cylinder bores and pistons. This will ultimately compromise the efficiency and safety of a vehicle’s brake system.

Classification of Brake Fluids

Brake fluids are non-petroleum and are classified by their physical properties – they are either glycol-ether based (DOT 3, 4, and 5.1) or silicone-based fluids (DOT 5). Quality standards refer to a brake fluid’s dry and wet boiling points. The wet boiling point, which is usually much lower, refers to the fluid’s boiling point after absorbing a certain amount of moisture. Non-hygroscopic fluids (e.g. silicone/DOT 5-based formulations) are hydrophobic and can maintain an acceptable boiling point over the fluid’s service life (although at the cost of potential phase separation/water pooling and freezing/boiling in the system over time – the main reason single-phase hygroscopic fluids are used).

Brake fluid boiling points.

It should also be noted that not all brake fluids are compatible with one another. The chart below shows which are interchangeable. Just remember that when you mix brake fluids with different specifications, you risk lowering the boiling point.

Closing Thoughts and Suggestions

Adding brake fluid is usually not part of routine vehicle maintenance. Low brake fluid typically means that your brake pads are low and need to be changed. If you notice a sudden drop in the brake fluid level, it means there’s a problem with your brake system, or that you need to change your brakes.

NEVER substitute any other fluid for brake fluid.

Old glycol brake fluid invariably contains moisture and should never be used, as it won’t be nearly as effective as an unopened bottle.

Changing glycol-based brake fluids renews the anti-corrosion protection that can only be accomplished with fresh brake fluid.

If you are thinking about making a change from glycol-based fluid to silicone brake fluid, make it part of a total brake system overhaul.

Finally, recognize the important role that your hydraulic brake system plays and change the brake fluid at least every other year.

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