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, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is not much of an issuecitation needed, but can be of serious concern in racecars or motorcyclescitation needed.
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)citation needed. The solution—after body repair—is to drain the brake system and rebuild all cylinders with new nitrile seals, then fill the system with the Girling-recommended fluid.
Boiling points
Minimal boiling points for these specifications are as follows:
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