Short answer
FMVSS 116 (49 CFR 571.116) is the Federal Motor Vehicle Safety Standard that sets minimum requirements for motor vehicle brake fluid in the United States. It defines the DOT 3, DOT 4, DOT 5 and DOT 5.1 grades, sets their boiling point and low-temperature viscosity limits, and dictates what has to appear on the container. The “DOT” on a bottle refers to this standard, issued under the US Department of Transportation and administered by NHTSA.
What the standard covers
FMVSS 116 applies to brake fluid intended for use in motor vehicle hydraulic brake systems and to the containers it is sold in. It is a performance standard: it does not prescribe a recipe, but every fluid sold under a DOT grade has to pass the same set of laboratory tests.
Beyond boiling point, the tests look at viscosity at 100 °C and minus 40 °C, pH, high-temperature stability, corrosion of brake-system metals, low-temperature fluidity and appearance, evaporation, water tolerance, compatibility with other brake fluids, oxidation resistance, the effect on rubber cups, and a stroking test that simulates repeated brake applications.
Dry and wet boiling points
The headline numbers are the equilibrium reflux boiling points (ERBP). The dry figure is measured on fresh fluid. The wet figure is measured after the fluid has been humidified under controlled conditions, until a reference fluid in the same chamber reaches about 3.7 percent water by weight.
Minimum dry/wet ERBP by grade: DOT 3, 205 °C (401 °F) dry and 140 °C (284 °F) wet; DOT 4, 230 °C (446 °F) and 155 °C (311 °F); DOT 5, 260 °C (500 °F) and 180 °C (356 °F). DOT 5.1 is a non-silicone fluid held to the DOT 5 limits. The wet number matters because glycol-based fluid absorbs moisture in service, and boiling fluid in a caliper produces vapor that compresses, giving a soft or sinking pedal.
Low-temperature viscosity
The standard caps how thick the fluid can be at minus 40 °C: 1,500 mm²/s for DOT 3, 1,800 mm²/s for DOT 4 and 900 mm²/s for DOT 5 grades. Thinner cold fluid moves faster through the small passages of ABS and stability-control modulators, which is one reason some vehicles call for a particular grade.
Color and labeling rules
FMVSS 116 assigns colors so incompatible fluids are easy to tell apart: DOT 3, DOT 4 and DOT 5.1 must be colorless to amber, DOT 5 silicone fluid must be purple, and hydraulic system mineral oil (a separate fluid type used in a few systems) must be green.
Containers have to show the grade, a statement that the fluid meets FMVSS 116, the packager, a lot number and the wet boiling point, and carry a warning to store the fluid only in its original container and keep it tightly closed to prevent moisture absorption. A DOT 5.1 label must say “DOT 5.1 NON-SILICONE BASE” so it is not mistaken for silicone DOT 5.
Frequently asked questions
Does FMVSS 116 say how often to change brake fluid?
No. It sets requirements for the fluid as sold. Service intervals come from the vehicle manufacturer, and some shops also test fluid for moisture or copper content.
Is FMVSS 116 the same as SAE J1703 or ISO 4925?
They are related but separate documents. SAE J1703 and ISO 4925 are industry brake fluid standards that cover similar ground; the federal DOT grades come from FMVSS 116.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].