DOT 5 is one of several North American designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.
DOT 5 is a silicone-based brake fluid (contains at least 70% by weight of a diorgano polysiloxane).
Unlike polyethylene glycol based fluids, Dot 5 is hydrophobic. An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range. Another property is that it does not damage paint.
Using DOT 5 in a DOT 3 or DOT 4 system without proper flushing will cause damage to the seals and cause brake failure. DOT 5 brake fluid is not compatible with anti-lock brake systems. DOT 5 brake fluid absorbs a small amount of air requiring care when bleeding the system of air.
Boiling points
Minimal boiling points for these specifications are as follows (wet boiling point defined as 3.7% water by volume):