DOT 4

DOT 4 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.

In the United States, all brake fluids must meet Standard No. 116; Motor vehicle brake fluids. Under this standard there are three Department of Transportation (DOT) minimum specifications for brake fluid. They are DOT 3, DOT 4, and DOT 5.1.

DOT 4, like DOT 3 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Fluids such as DOT 3, 4, and 5.1 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 usually not much of an issue as the brakes are generally not used aggressively, but can be of serious concerns in police vehicles, racecars or motorcycles due to the higher heat generated during their much more aggressive braking, and trucks where higher heat can be generated while braking hard and fully loaded.

While a vehicle that uses DOT 3 may also use DOT 4 or 5.1 if the elastomers in the system accept the borate compounds that raise the boiling point, (a temperature upgrade) a vehicle that requires DOT 4 might boil the brake fluid if a DOT 3 (a temperature downgrade) is used. Additionally, these polyglycol ether based fluids cannot be mixed with DOT 5.0, which is silicone based.

As of 2006^[update], most cars produced in the U.S. use DOT 4 brake fluid.

Minimal boiling points for these specifications are as follows (wet boiling point defined as 3.7% water by volume):

The MSDS for one brand lists the general contents as "Blend of polyglycol ethers, glycol ether borate esters and polyglycols with added corrosion and oxidation inhibitors.

One particular brand of DOT 4 brake fluid lists the following ingredients on its MSDS:

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