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Brake fluid

Car culture term

Brake fluid
Photo: Alf van Beem / Wikimedia Commons · CC0

What is Brake fluid?

Hydraulic fluid that transfers force from the brake pedal to the brake calipers. Must be replaced periodically due to moisture absorption.

Brake fluid is hydraulic fluid that transfers force from the brake pedal to the brake calipers. The fluid is glycol-ether based (DOT 3, DOT 4, DOT 5.1) or silicone-based (DOT 5). The fluid must compress under braking pressure to transfer force; air bubbles or moisture-saturated fluid cannot compress effectively, causing soft brake pedal and reduced braking performance.

Brake fluid is hygroscopic (absorbs moisture from the atmosphere). Over time, the fluid absorbs water from air entering through brake reservoir vents and from brake hoses (which slowly seep moisture). The water reduces the boiling point and creates corrosion. Modern brake fluid specifications (DOT 4, DOT 5.1) tolerate more moisture than older DOT 3, but all glycol-ether fluids absorb moisture and require periodic replacement.

The recommended replacement interval is typically 2-3 years (sometimes 24,000 miles whichever comes first). The replacement involves bleeding the fluid from each brake caliper while keeping the master cylinder full. The work can take 1-2 hours and is one of the more important fluid changes for brake system reliability. Track-driven cars often replace fluid more frequently (every 6 months to 1 year) due to the heat cycles experienced during track use. The cost is typically $80-$200 in parts and labor.

Related terms

Coolant flushOil change intervalBig brake kit

Frequently asked questions

Why does brake fluid need replacement?
Brake fluid absorbs moisture from the atmosphere over time. The water reduces the fluid boiling point (causing brake fade under heat) and accelerates corrosion of brake system components. Periodic replacement ensures braking performance and prevents corrosion-related component failure. Most manufacturers recommend replacement every 2-3 years.
What is the difference between DOT 3, 4, and 5?
DOT 3 is older glycol-ether fluid with lower boiling point and less moisture tolerance. DOT 4 is newer glycol-ether with higher boiling point and better moisture tolerance. DOT 5 is silicone-based, doesn't absorb moisture, but cannot be mixed with other DOT fluids. DOT 5.1 is glycol-ether like DOT 4 but with even higher performance. Most modern cars use DOT 3 or DOT 4; DOT 5.1 is used in performance applications.
Can I mix brake fluids?
DOT 3 and DOT 4 can be mixed (they are chemically compatible). DOT 5 (silicone-based) cannot be mixed with any glycol-ether fluid. DOT 5.1 (despite the similar number) is glycol-ether and can be mixed with DOT 3 and DOT 4. Always verify the fluid type and specification before mixing.
How often should track-driven cars change brake fluid?
More frequently than street cars. Track use generates extreme brake temperatures that accelerate fluid degradation. Recommendations vary: every 6 months to 1 year for street/track cars, before each track event for serious enthusiasts, or after every 3-5 track days. Performance brake fluid (Motul RBF600, Castrol SRF, ATE TYP200) is sometimes used for higher boiling point.
Can I bleed brakes myself?
Yes, with proper tools. A pressure brake bleeder or a manual two-person process with a friend pumping the brake pedal. The work requires keeping the master cylinder full and bleeding each caliper in proper sequence (typically furthest from master cylinder first). DIY brake bleeding is achievable but requires care to avoid air ingestion.
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