Q: How Does Brake Fluid Work?

Brake fluid is a specialized type of hydraulic fluid whose properties are designed to provide pressure transfer to operate the brake while dissipating the heat developed during braking. Brake fluid is hygroscopic, which means it can absorb water, so it should always be kept in a sealed container when stored and a sealed system when installed in your vehicle. Since fluids cannot be compressed, they create a reaction within a sealed system according to a principal of science known as Pascal’s law (Force = Area x Pressure). By using this principle we can take a relatively small piston size inside the master cylinder and by depressing the pedal, exert physical force from the driver which will develop pressure inside the brake system. The pressure will be constant on all surfaces that it acts upon equally. It exerts this pressure through the brake system and when it contacts the surface area on the caliper or brake wheel cylinder, the pressure against those pistons amplifies the output force applied to the brakes themselves. The initial force of the driver is also assisted or amplified by a brake booster which can be vacuum, hydraulic, or electrically assisted. When the pedal is released, springs or seals allow the fluid pressure to decrease, which allows the brakes to release.

Think of blowing up a balloon: the air pressure pushes equally on the entire inside of the balloon’s surface. If you then push on the balloon, you compress it and force the balloon to squeeze out sideways. When you release the pressure, the balloon returns to its original shape. Air being compressed is a somewhat similar concept, yet air can be compressed while liquids cannot. Imagine what happens if you fill a water bottle completely to the top, put the lid back on tightly, and then step on the bottle really fast and hard. The bottle will resist until the force is great enough to pop the lid off or the bottle breaks.

Having the proper brake fluid type for your vehicle allows it to absorb the heat without boiling and push or apply adequate pressure to the brake pads or shoes to stop your vehicle. In the event of a brake system failure your pedal may go too low or even to the floor, requiring you to pump it several times to attempt to brake. Proper care of brake components and fluid will keep your brake system in top performance, allowing for safer stops. Make sure to have the fluid level and condition checked on a regular basis and have the fluid changed or flushed on a regular basis every couple of years, or better yet have this done when your brakes are replaced.