Fleet & Auto Reference Library · Fluids & Specs
Brake Fluid — DOT 3, DOT 4, DOT 5 & DOT 5.1
The DOT brake fluid grades defined by FMVSS 116, their federal minimum boiling points, which grades can and cannot be mixed, and what to check before selling a fluid for a hydraulic brake system.
Safety-critical sheet, not yet signed off by an expert reviewer. Use it to understand the topic, then take every value and decision from the governing standard, the manufacturer’s instructions and a qualified person.
What it is
Hydraulic brake systems on cars, light trucks and many medium-duty trucks use brake fluid graded under FMVSS No. 116 (49 CFR 571.116). The standard sets minimum boiling points, viscosity, corrosion, rubber-compatibility and other requirements, and labeling rules for containers. DOT 3, DOT 4 and DOT 5.1 are glycol-ether based (DOT 4 and 5.1 add borate esters). They absorb water from the air over time, which lowers their boiling point. DOT 5 is silicone based (SBBF) and does not absorb water. Air-brake trucks do not use brake fluid in the air system; the fluid applies only to hydraulic brake circuits.
How to identify
The container must state the DOT grade. Check the vehicle's master cylinder cap, owner's manual or service information for the required grade. DOT 5 silicone fluid is often dyed purple; glycol fluids are typically clear to amber — but color is not a reliable identifier, and the label and the vehicle specification govern.
Federal minimum boiling points
Values are the minimum equilibrium reflux boiling points (ERBP) in FMVSS 116, S5.1.1 (dry) and S5.1.2 (wet). Containers must also show the fluid's own minimum wet boiling point.
| Grade | Base | Min. dry ERBP | Min. wet ERBP | Mixes with |
|---|---|---|---|---|
| DOT 3 | Glycol ether | 205 °C (401 °F) | 140 °C (284 °F) | DOT 4, DOT 5.1 |
| DOT 4 | Glycol ether / borate ester | 230 °C (446 °F) | 155 °C (311 °F) | DOT 3, DOT 5.1 |
| DOT 5 | Silicone (SBBF) | 260 °C (500 °F) | 180 °C (356 °F) | Do not mix with glycol fluids |
| DOT 5.1 | Glycol ether / borate ester (non-silicone) | See 49 CFR 571.116 | See 49 CFR 571.116 | DOT 3, DOT 4 |
Key points
- Use the grade the vehicle maker specifies. A higher-grade glycol fluid can usually be used where DOT 3 is specified if the maker allows it; never substitute DOT 5 for a glycol fluid, or the reverse, without a full system conversion approved by the vehicle maker.
- DOT 5 and DOT 5.1 are different products. The similar names cause expensive mistakes.
- Glycol fluids pick up moisture once opened. Sell sealed containers, keep caps tight, and do not reuse fluid.
- Glycol brake fluids damage paint. Clean spills immediately.
- Mineral-oil (LHM) systems used on some vehicles and equipment are not DOT fluid systems; never put DOT fluid in them, or mineral oil in a DOT system.
- Brake fluid is a high-risk item: a wrong fluid can swell seals and cause brake failure.
Common mistakes
- Topping off a DOT 3 system with DOT 5 because "5 is higher than 3."
- Confusing DOT 5.1 with DOT 5.
- Selling an opened or partly used container.
- Using power-steering or hydraulic oil in a brake reservoir.
Governing standards: FMVSS No. 116, 49 CFR 571.116 (motor vehicle brake fluids). Vehicle maker's service information for the required grade and service interval.
Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.