Short answer
DOT 3 is a brake fluid grade defined by FMVSS 116, the US federal brake fluid standard. It is a glycol-ether based fluid that must have a dry boiling point of at least 205 °C (401 °F) and a wet boiling point of at least 140 °C (284 °F). It has long been the factory fill in many cars and light trucks, though many newer vehicles specify DOT 4.
What DOT 3 is made of
DOT 3 fluids are built mostly from glycol ethers, with additives to protect brake-system metals and rubber seals. Like all glycol-based brake fluids, DOT 3 is hygroscopic: it pulls moisture from the air through hoses, seals and the reservoir vent.
That moisture absorption is by design. Water is dispersed through the fluid instead of collecting in low spots such as caliper bores, where free water would freeze or corrode parts. The trade-off is that the boiling point falls as water content rises.
The numbers DOT 3 has to meet
FMVSS 116 sets DOT 3 minimums of 205 °C dry and 140 °C wet equilibrium reflux boiling point, and a maximum viscosity of 1,500 mm²/s at minus 40 °C. These are floors, not typical values; many fluids sold as DOT 3 exceed them, and the wet boiling point must be printed on the container.
The gap between the dry and wet figures shows why age matters. A fluid that starts well above the dry minimum can drift toward the wet figure after a few years of moisture absorption, which reduces the margin against fade during long downhill braking or towing.
Where DOT 3 is specified
DOT 3 has been common in domestic passenger cars and light trucks for decades. The reservoir cap or owner’s manual names the grade; follow it, because ABS and stability-control systems are calibrated around fluid properties, including cold viscosity.
Substituting another grade
DOT 3, DOT 4 and DOT 5.1 are all glycol-based and are formulated to mix without separating. Many manufacturers allow DOT 4 where DOT 3 is listed, since DOT 4 minimums are higher, but a few systems have specific requirements, so check the manual first. Going the other way, putting DOT 3 into a system that calls for DOT 4 or DOT 5.1, is not appropriate.
DOT 3 must never be mixed with DOT 5 silicone fluid. The two do not blend, and FMVSS 116 requires silicone DOT 5 to be dyed purple partly to prevent that mistake.
Frequently asked questions
Can DOT 3 damage paint?
Glycol-based brake fluids, including DOT 3, can soften or lift automotive paint. Wipe spills promptly and rinse the area with water.
Why does an opened bottle of DOT 3 go bad?
Once opened, the fluid absorbs moisture from the air. The FMVSS 116 label warning to keep the container tightly closed exists for that reason; many shops discard partly used bottles rather than storing them.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].