DOT 5.1
DOT 5.1 is one of several designations of brake fluid denoting a particular mixture of chemicals imparting specified ranges of boiling point.
All brake fluids must meet federal standard #116. Under this standard there are three Department of Transportation (DOT) minimal specifications for brake fluid. They are DOT 3, DOT 4, and DOT 5.1.
DOT 5.1, like DOT 3 and DOT 4, is a polyethylene glycol based fluid (contrasted with DOT 5 which is silicone based). Fluids such as DOT 5.1 are hygroscopic and will absorb water from the atmosphere. This degrades the performance of the fluid by drastically reducing its boiling point. In a passenger car this is not much of an issue, but can be of serious concerns in racecars or motorcycles.
Boiling Points
MINIMAL boiling points for these specifications are as follows:
| Dry Boiling Point | Wet Boiling Point | |
|---|---|---|
| DOT 3 | 205°C (401°F) | 140°C (284°F) |
| DOT 4 | 230°C (446°F) | 155°C (311°F) |
| DOT 5 | 260°C (500°F) | 180°C (356°F) |
| DOT 5.1 | 270°C (518°F) | 191°C (375°F) |