Replacing brake fluid is a critical maintenance task that involves flushing the old, moisture-contaminated fluid from the hydraulic system and replacing it with new fluid. While a DIY job for those with moderate mechanical skill, it requires specific tools and careful attention to safety and procedure to avoid introducing air into the system, which can lead to brake failure.
The primary reason for replacement is that brake fluid is hygroscopic, meaning it absorbs moisture from the air over time. This moisture lowers the fluid's boiling point. Under heavy braking, the heat generated can cause the water in the fluid to vaporize, creating compressible air bubbles in the lines and resulting in a spongy brake pedal and dangerously reduced stopping power. Most manufacturers recommend a flush every 2-3 years or 30,000 miles, but you should always consult your owner's manual.
Tools and Materials You'll Need:
- New brake fluid (check your manual for the correct DOT specification, like DOT 3, DOT 4, or DOT 5.1)
- A wrench to fit your vehicle's brake bleeder valves
- A clear plastic hose that fits snugly over the bleeder valve
- A clean collection bottle for the old fluid
- A friend to help pump the brake pedal
- Jack and jack stands to safely lift and secure the vehicle
- Safety glasses and gloves
The Step-by-Step Process:
- Preparation: Locate the master cylinder reservoir under the hood and clean its cap area thoroughly to prevent contamination. Siphon out most of the old fluid from the reservoir and refill it with new fluid. This prevents the system from running dry.
- Lift and Secure: Safely jack up the car and support it with jack stands. Never rely on a jack alone.
- The Flushing Order: The correct sequence is crucial. Always start with the wheel farthest from the master cylinder and work your way to the closest. For most cars, this is: Rear Passenger Side -> Rear Driver Side -> Front Passenger Side -> Front Driver Side.
- Bleeding the Brakes: Have your helper slowly pump the brake pedal a few times and then hold firm pressure. With pressure applied, open the bleeder valve on the first wheel about a quarter-turn. Old fluid will flow through the tube into your collection bottle. Once the flow stops, close the valve before your helper releases the pedal. Repeat this process at each wheel until the fluid coming out is clean and free of air bubbles, topping off the master cylinder frequently to never let it run dry.
Brake fluid is corrosive to paint, so wipe up any spills immediately with water. Dispose of the old fluid at an auto parts store or a hazardous waste facility; never pour it down the drain. If you are unsure about any step, seeking professional service is the safest choice.
| Common DOT Brake Fluid Specifications | Typical Dry Boiling Point | Typical Wet Boiling Point | Primary Use |
|---|
| DOT 3 | 401°F (205°C) | 284°F (140°C) | Common in older passenger vehicles |
| DOT 4 | 446°F (230°C) | 311°F (155°C) | Common in modern cars, higher performance |
| DOT 5.1 | 500°F (260°C) | 356°F (180°C) | High-performance and ABS-equipped vehicles |
| DOT 5 (Silicone) | 500°F (260°C) | Does not absorb water | Classic cars, not for ABS systems |
