DOT brake fluid, and why DOT 5 is the odd one

What a DOT number sets

A set of minimum properties that a fluid must meet, among them a dry boiling point and a wet boiling point. It is a floor rather than a description: two fluids carrying the same designation can differ substantially, and makers publish their own measured figures alongside.

The two boiling points, and why both are published

The dry figure is measured on fresh fluid. The wet figure is measured on fluid that has absorbed a defined amount of water, and it is always the lower of the two. Glycol-based brake fluid is hygroscopic — it draws moisture from the air through seals and hoses over time — so a system that has not been bled in years is operating nearer its wet figure than its dry one. That is why brake fluid has a service interval, published by the machine’s manufacturer.

The mixing rule, which is where the names mislead

DOT 3, DOT 4 and DOT 5.1 are glycol-based and can be mixed, with the resulting system taking on the properties of the lowest specification present. DOT 5 is silicone-based, is not hygroscopic, and must not be mixed with any of the others. The names are adjacent and the products are not, and DOT 5.1 being glycol-based while DOT 5 is not is the single most misread pair of designations on this shelf.

The same three letters elsewhere

DOT also appears on helmets sold to a United States standard, and that is a different document entirely. A sigil is not a standard; the document behind it is.