Brake Fluid Service

The physics are straightforward. Brake fluid absorbs moisture continuously through the rubber portions of brake hoses and through the reservoir vent. The longer it sits in the system, the more moisture it contains. The more moisture it contains, the lower its boiling point. The lower its boiling point, the less thermal capacity it has during hard or repeated braking. At some point — usually during a long downhill or emergency stop — the fluid reaches its reduced boiling point and vaporizes. Vapor compresses. Brake fluid does not. The pedal fades or disappears.

DOT 3, DOT 4, and DOT 5.1 are all glycol-based fluids and are compatible with each other. The higher the number, the higher the boiling point. DOT 3 has a dry boiling point of 401 degrees Fahrenheit. DOT 4 is 446 degrees. DOT 5.1 is 500 degrees. Always use the fluid type specified by the manufacturer — many European vehicles require DOT 4 minimum. DOT 5 — not DOT 5.1 — is silicone-based. It is NOT compatible with glycol-based fluids. DOT 5 is used almost exclusively in military vehicles and some collector cars. Never put DOT 5 in a vehicle designed for DOT 3 or DOT 4. The system will fail.

A brake fluid test strip or refractometer measures the moisture content of the fluid. You can also use a boiling point tester that heats a small sample and reads the actual boiling point. Any result showing moisture content above 3 percent or a boiling point that has dropped significantly from the dry specification means the fluid is due for replacement. Every two years, regardless of mileage, brake fluid replacement is justified on safety grounds alone. It also protects the internal metal components of calipers, wheel cylinders, and the master cylinder from the corrosion that moisture-contaminated fluid causes over time.

After any brake fluid service, air must be purged from the system. Bleed in the correct sequence — typically starting at the wheel farthest from the master cylinder and working toward the closest. Right rear, left rear, right front, left front on most vehicles. Use a pressure bleeder, vacuum bleeder, or manual two-person method. Never let the master cylinder reservoir run dry during bleeding — sucking air into the master cylinder means starting the process over. On vehicles with ABS, some systems require a scan tool to cycle the ABS pump and valves to purge air trapped inside the hydraulic control unit.