Brake Fluid
Engine Oil & Fluids · 6 specs
Direct Answer
DOT Brake Fluid (standardized under US FMVSS 116, SAE J1703/J1704, and ISO 4925) defines the hydraulic fluid specifications for automotive braking systems. Governing Dry Boiling Points (fresh fluid) and Wet Boiling Points (3.7% water contamination absorbed over time) across glycol-ether fluids (DOT 3, DOT 4, DOT 5.1) and silicone-based fluid (DOT 5).
Source: US DOT FMVSS 116 / SAE J1703 & J1704 / ISO 4925 (Road vehicles — Specification of non-petroleum-base brake fluids) · Last reviewed Aug 28, 2026
Specs
| Standardization | FMVSS 116 / SAE J1703 (DOT 3), SAE J1704 (DOT 4), ISO 4925 Class 3/4/6/7 |
| DOT 3 Minimum Boiling Points | Dry: 205°C (401°F) | Wet (3.7% H₂O): 140°C (284°F) — Standard domestic vehicles |
| DOT 4 Minimum Boiling Points | Dry: 230°C (446°F) | Wet: 155°C (311°F) — Modern European and Japanese passenger cars with ABS/ESP |
| DOT 4 Class 6 (Low Viscosity) | Low viscosity at -40°C (max 750 mm²/s vs 1500 mm²/s standard) for rapid ABS/ESP active brake pulsing in freezing winters |
| DOT 5.1 (High Performance Glycol) | Dry: 260°C (500°F) | Wet: 180°C (356°F) — Glycol-ether base compatible with DOT 3 and DOT 4 systems |
| DOT 5 (Silicone-Based — Purple) | Dry: 260°C (500°F) | Hydrophobic silicone base (NEVER mix with DOT 3/4/5.1; used in classic cars and military Harley-Davidsons) |
Hygroscopic Nature and Vapor Lock Danger
Glycol-ether brake fluids (DOT 3, 4, 5.1) are hygroscopic, naturally absorbing moisture through microscopic pores in rubber brake hoses. After 2–3 years, fluid absorbs sim 3.7% water, dropping its boiling point to the Wet Boiling Point. Under heavy mountain downhill braking, water in contaminated fluid boils into steam gas bubbles (100°C): because gas is compressible, the brake pedal drops to the floor with zero braking pressure (vapor lock).