Imagine pressing your car’s brake pedal and nothing happens. It's a scary thought. But it's a real possibility if you neglect your vehicle’s brake fluid.
Brake fluid is the stuff that turns the pressure of your foot on the brake pedal into stopping power at the wheels. And similar to other fluids in your car, like oil and coolant, it degrades over time, eroding its ability to carry out this critical function. That's why checking and changing your brake fluid at regular intervals is one of the most important car maintenance tasks.
But before you dive into changing your brake fluid or even just topping off the reservoir, know that not all brake fluids are created equal. Some are safe to put in your car. Others can wreak havoc on the system.
That's why we've put together this detailed guide. It walks you through everything you need to know about brake fluid.
What Is Brake Fluid?
Brake fluid is a type of hydraulic fluid. It runs in hoses and lines between the brake calipers at each wheel and the brake master cylinder, typically located under the hood on the firewall. When you press the brake pedal, the brake booster amplifies this force, and the brake master cylinder translates it to the fluid. This results in hydraulic pressure at the calipers, which squeezes the brake pads against the rotors, slowing the wheels and bringing the car to a halt.
To do this well, brake fluid must possess several key characteristics. Most important, it needs to be non-compressible. This ensures the direct and efficient transmission of hydraulic pressure to the calipers, giving the brakes a solid and responsive feel. The fluid also needs a high boiling point to withstand the heat generated during braking. Otherwise, the fluid could vaporize, leading to brake failure.
Brake fluid also contributes to the overall health of the braking system. It keeps moving parts like the master cylinder lubricated and conditions the rubber seals and hoses, keeping them pliable and strong. That not only ensures their smooth operation but prevents leaks, and protects the metal components within the system from corrosion, prolonging their life.
The Four Types of Brake Fluid
Brake fluid isn't a one-size-fits-all solution. There are multiple types available, each with distinct properties. And because brake fluid is such an essential component of vehicle safety, its various formulations are categorized and regulated by the U.S. Department of Transportation (DOT).
The most common type is DOT 3. It's a clear or honey-colored glycol-ether solution with properties that make it well-suited for most normal driving. DOT 4, often found in European cars, is similar but includes additives that raise its boiling point, making it better in more demanding conditions, such as towing or performance driving.
Both of these glycol-based fluids are hygroscopic, meaning they absorb moisture. This prevents any water that enters the braking system from corroding its internal components. Over time, though, water absorption lowers the brake fluid's boiling point and reduces its effectiveness. This necessitates changing glycol-based brake fluid regularly -- typically every two to three years -- to maintain braking performance.
DOT 3 has a boiling point of 401 degrees Fahrenheit before it has absorbed any moisture. This is referred to as its dry boiling point. DOT 4 has a dry boiling point of 446 degrees Fahrenheit.
Unlike those glycol-based fluids, DOT 5 is silicone-based and usually appears purple. It boasts a very high dry boiling point of 500 degrees Fahrenheit as well as a non-hygroscopic nature, meaning it does not absorb water. The latter helps it go longer between changes, making it a popular choice for cars that are driven infrequently, such as classics. However, it's critical to note that DOT 5 fluid is not compatible with anti-lock braking systems (ABS), nor should it be mixed with glycol-based fluids, such as DOT 3 or 4.
DOT 5.1 bridges the gap between the other types. It's glycol-based like DOT 3 and 4, but it boasts a boiling point as high as DOT 5. It's also compatible with most modern braking systems, including vehicles equipped with ABS. These qualities make it a popular choice for high-performance and heavy-duty vehicles.
How To Choose the Right Brake Fluid
The first step when choosing the right brake fluid is consulting your vehicle's owner's manual. There you'll find the type specified by the manufacturer for your car. The manual should also indicate the recommended interval for brake fluid changes.
Your typical driving habits and conditions should also factor into your selection. For example, if your commute involves lots of stop-and-go traffic or mountain roads, or if you do lots of towing and hauling, your brakes are working harder and generating more heat than normal. In such cases, a brake fluid that can withstand higher temperatures, such as DOT 4 or DOT 5.1, would be a beneficial upgrade.
For classic cars that see infrequent use, a non-hygroscopic fluid like DOT 5 can be a smart choice. It can go far longer between changes without a degradation in performance. Remember, though, that silicone-based fluids like DOT 5 are not compatible with ABS or other fluid types.
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