DOT 3

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

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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: