DOT 3 fluid is a glycol-based motor-vehicle brake fluid defined by FMVSS 116 with a minimum dry boiling point of 205 °C (401 °F).
Pop the hood, crack the cap, and you’ll see it on the reservoir: DOT 3. That label isn’t a brand; it’s a performance grade for hydraulic brake fluid set by federal standards. It tells you the fluid’s base chemistry, heat tolerance, cold-weather flow, and the labeling rules on the bottle. If your cap says DOT 3, stick with it unless your owner’s manual says otherwise.
DOT 3 Brake Fluid: Meaning And Specs
DOT 3 is a polyglycol-ether brake fluid used in millions of passenger cars and light trucks. The grade is defined in the U.S. Federal Motor Vehicle Safety Standard FMVSS 116, which sets minimum laboratory targets for things like boiling point, viscosity, corrosion control, and color. In short, DOT 3 must reach a dry boiling point of 205 °C (401 °F) and hold at least 140 °C (284 °F) after aging with moisture. Kinematic viscosity can’t exceed 1,500 mm²/s at −40 °C, and the fluid appears colorless to amber when fresh. On the bottle you’ll also see warnings to keep the fluid clean and dry, because water and dirt can ruin brakes fast.
Specs You’ll See On The Label
- Dry ERBP (new fluid from a sealed container): 205 °C (401 °F) or higher for DOT 3.
- Wet ERBP (aged, moisture-exposed fluid): 140 °C (284 °F) or higher for DOT 3.
- Viscosity limit at −40 °C: no more than 1,500 mm²/s for DOT 3.
- pH window and corrosion limits to protect steel, aluminum, copper, and brass parts.
- Color cue: DOT 3/4/5.1 are colorless to amber; DOT 5 silicone is purple.
- Storage notes: keep tightly closed, clean, and dry; never pour back used fluid.
| Measure | DOT 3 | DOT 4 |
|---|---|---|
| Dry boiling point | 205 °C (401 °F) | 230 °C (446 °F) |
| Wet boiling point | 140 °C (284 °F) | 155 °C (311 °F) |
| Max viscosity at −40 °C | 1,500 mm²/s | 1,800 mm²/s |
| Appearance | Colorless to amber | Colorless to amber |
See the official FMVSS 116 listing for full definitions, test methods, and labeling rules.
DOT 3 Vs DOT 4 And DOT 5: What Changes
All DOT grades are measured with the same government tests, but the targets move. DOT 4 raises the boiling points and allows slightly thicker fluid at deep-freeze temperatures. DOT 5 is silicone-based and purple, aimed at specific applications. Most modern road cars specify DOT 3 or DOT 4; some high-performance or heavy-duty systems call for DOT 5.1, which is a glycol fluid that reaches DOT 5-level heat targets without the silicone base.
How DOT 3 Works Inside Your Brakes
Press the pedal and the master cylinder pushes DOT 3 through steel lines and flexible hoses. The fluid transmits force to caliper pistons or wheel cylinders with minimal compression, so pedal travel turns into clamp force at the pads. When fluid boils, vapor bubbles form and compress easily, which kills pedal feel. Because DOT 3 absorbs small amounts of moisture from air, the boiling point drops over time. That’s why a fresh, sealed bottle tests higher than old fluid drawn from a car that’s lived through winters and humidity. The standard includes procedures that simulate age and water exposure to verify performance stays above the required floor.
Service Life, Storage, And Testing
Brake feel fades for many reasons, but old fluid is an easy one to prevent. Use a sealed container, cap it right after pouring, and don’t store a half-used bottle for long. On the car, follow your maker’s change interval and use the specified grade. Shops can check moisture content or boiling point with simple tools, which helps decide if it’s time to flush. If you wrench at home, watch for cloudiness, rust tint, or dark debris when you crack a bleeder—those are clues the fluid is past its best.
Why Moisture Matters
Glycol-based fluid pulls in water slowly through hoses, seals, and the reservoir vent. A little water lowers the boiling point and can push corrosion along inside calipers, wheel cylinders, and ABS modulators. That’s also why storage notes on the bottle stress clean and dry handling, and why techs avoid leaving a container uncapped on a bench.
Can You Mix DOT 3 And DOT 4?
Yes, DOT 3 and DOT 4 are glycol-based and mix safely, but the system performs to the lower grade you pour. Topping a DOT 3 system with DOT 4 is common in a pinch; switching entirely is fine if the manual allows it. Do not add DOT 5 silicone to any system filled with glycol fluid. If a previous owner used DOT 5, a complete teardown and cleanout is the only safe path before moving back to glycol fluids. DOT 5.1 is different—it’s glycol-based and can mix with DOT 3 or DOT 4 while hitting higher heat targets.
Quick Compatibility Rules
- DOT 3 ↔ DOT 4: compatible; performance defaults to the lower grade.
- DOT 3/4 ↔ DOT 5.1: compatible; all are glycol-based.
- DOT 5 (silicone) with any glycol fluid: don’t mix; flush and rebuild if switching.
About DOT 5 Silicone
Silicone fluid doesn’t absorb water and is dyed purple for easy ID. It’s used in some specialty vehicles and restorations. Because it doesn’t blend with glycol, mixing creates pockets that can trap moisture and cause corrosion or spongy feel. Unless your system was built for it and filled from dry, skip DOT 5.
Signs Your DOT 3 Needs Attention
Spongy feel, a longer pedal, or repeated fade after hills often traces back to old, wet fluid. Leaks at calipers, wheel cylinders, or the master cylinder also contaminate fluid with air and debris. Any time a brake hydraulic part is opened, plan on a full bleed with fresh DOT 3 that meets the standard on your cap.
| Symptom | Likely Cause | What To Do |
|---|---|---|
| Soft pedal after long hill | Moisture-lowered boiling point | Flush with fresh DOT 3 |
| Pedal slowly sinks at stop | Internal master cylinder leak | Replace master; bleed system |
| Uneven pull while braking | Sticking caliper or contaminated fluid | Service caliper; flush |
| Dark, rusty fluid at bleeder | Age, moisture, or corrosion | Full flush and inspection |
| Firm pedal, poor stopping | Pad/rotor issue, not fluid | Inspect friction parts |
For lab test detail, NHTSA’s TP-116-04 test procedure explains how boiling points and other checks are run.
How To Top Off Or Flush Safely
Work on a cool car. Cover painted panels; DOT 3 can strip finish fast. Clean the reservoir cap area so grit can’t fall in. Use only new fluid from a sealed bottle. Fill to the mark, not above it. If you’re flushing, start at the wheel farthest from the master cylinder and keep the reservoir from running dry. Finish with a firm pedal check and a road test in a safe area.
DIY Bleed Order Tips
- Rear right → rear left → front right → front left is common on left-hand-drive cars; some layouts differ.
- Keep the level up; pulling air into the master adds time and frustration.
- Use clear hose on the bleeder to spot bubbles and debris.
- ABS units may need a scan-tool routine; follow the service manual for that step.
When DOT 3 Still Makes Sense
Plenty of daily drivers call for DOT 3 and run perfectly with it. If your manual specifies DOT 3 and you don’t tow, track, or live in the mountains, there’s no need to chase a higher grade. Focus on fresh, uncontaminated fluid, correct bleeding, and good rubber hoses; those choices pay off more than chasing a number on the label.