Brake Fluid Reference
DOT ratings explained, compatibility rules, boiling points, and a brake symptom guide. Using the wrong fluid or letting it degrade can cause complete brake failure.
DOT Fluid Comparison
| Grade | Dry BP | Wet BP | Absorbs Water |
|---|---|---|---|
| DOT 3 | 205°C | 140°C | Yes |
| DOT 4 | 230°C | 155°C | Yes |
| DOT 5 | 260°C | 180°C | No |
| DOT 5.1 | 260°C | 180°C | Yes |
Dry BP = fresh fluid. Wet BP = fluid after absorbing 3.7% water (FMVSS 116 spec). Click a row for details.
Recommended Flush Intervals
| Track / Race | Every session or every 6 months |
| Sport / daily street | Every 1–2 years |
| Casual / weekend use | Every 2 years |
| Long-term storage | Before returning to service |
| Any (warning signs) | Immediately |
Brake Symptom Guide
Spongy lever / pedal
Cause: Air in system or degraded fluid (high moisture content)
Fix: Bleed brakes. If fluid is dark or old, flush completely.
Lever goes to the bar
Cause: Air, blown master cylinder seal, or leaking caliper
Fix: Inspect master cylinder and calipers. Bleed and inspect seals.
Brake fade on track / descent
Cause: Fluid has boiled (vapor lock) — wet BP exceeded
Fix: Flush with fresh high-spec fluid (DOT 4+ or 5.1). Bleed fully.
Fluid is dark brown or black
Cause: Contaminated with moisture, metal particles, or age
Fix: Full flush and bleed immediately. Inspect master cylinder bore.
Dragging brakes
Cause: Caliper piston not returning — swollen seals (wrong fluid?)
Fix: Check for DOT 5 / DOT 3/4 mixing. Rebuild or replace calipers.
Safety Rules
• NEVER mix DOT 5 (silicone) with DOT 3/4/5.1 (glycol) — the resulting sludge causes seal failure and brake loss.
• Higher DOT number = higher boiling point, but doesn't mean you can flush less often (glycol fluids still absorb moisture).
• Always use fresh, sealed fluid. Opened bottles absorb moisture from air — discard after 1 year.
• Brake fluid strips paint — clean spills immediately with water.