DOT 3

DOT 3 is one of several designations of brake fluid denoting a particular mixture of chemicals imparting specified ranges of boiling point.

Additional recommended knowledge

How to ensure accurate weighing results every day?

Weighing the right way

How to quickly check pipettes?

In the United States, 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 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 by drastically reducing its boiling point. In a passenger car this is not much of an issue, but can be of serious concern in racecars or motorcycles.

As of 2006, most cars produced in the U.S. use DOT 3 brake fluid.

Quirks

Girling brakes, which were widely used in British cars in the mid-20th century, use nitrile seals, which are degraded by DOT-3 brake fluid. Many owners of British cars had no problems with their brakes as long as their brakes were serviced by a British-car dealer who used the Girling- recommended "Castrol Golden Amber" brake fluid, but at some indeterminate time after allowing a non-specialist mechanic to top up oil and "all fluids," felt their brake pedal go to the floor without stopping the car. The problem: the generalist mechanic used DOT-3 brake fluid in all cars and rarely sees a British car (twice). The solution—after body repair—is to drain the brake system and rebiuld all cylinders with new nitrile seals, then fill the system with the Girling-recommended fluid.

This property of the U.S.-standard brake fluid and U.K.-standard brakes is usually learned by experience—that is, the hard way.

Boiling points

Minimal boiling points for these specifications are as follows: