In the United States, all brake fluids must meet Standard No. 116; Motor vehicle brake fluid. Under this standard there are four Department of Transportation (DOT) minimal specifications for brake fluid. They are DOT 3, DOT 4, DOT 5 and DOT 5.1.
DOT 3, like DOT 4 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5, which is silicone-based). Fluids such as DOT 3 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is not much of an issue, but can be of serious concern in police vehicles, racecars or motorcycles.
DOT3 has been all but replaced with the superior DOT4 as there is little cost difference between the two.
Jan. 4th, 2018