DOT 4 is one of several designations of automotive brake fluid

Brake fluid

Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure...

, denoting a particular mixture of chemicals imparting specified ranges of boiling point

Boiling point

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

.

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

United States Department of Transportation

The United States Department of Transportation is a federal Cabinet department of the United States government concerned with transportation. It was established by an act of Congress on October 15, 1966, and began operation on April 1, 1967...

 (DOT) minimum specifications for brake fluid. They are DOT 3

DOT 3

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

, DOT 4, and DOT 5.1

DOT 5.1

DOT 5.1 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point....

.

DOT 4, like DOT 3 and DOT 5.1, is a polyethylene glycol

Polyethylene glycol

Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...

-based fluid (contrasted with DOT 5

DOT 5

DOT 5 is one of several designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.DOT 5 is a silicone-based brake fluid ....

 which is silicone

Silicone

Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....

-based). Fluids such as DOT 4 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 so hard, but can be of serious concerns in racecars or motorcycle

Motorcycle

A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...

s due to their much more aggressive braking.

, most cars produced in the U.S. use DOT 4 brake fluid.

Boiling points

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

Boiling point

Boiling point

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

 ranges

	Dry boiling point	Wet boiling point
DOT 3	205 °C (401 °F)	140 °C (284 °F)
DOT 4	230 °C (446 °F)	155 °C (311 °F)
DOT 5	260 °C (500 °F)	180 °C (356 °F)
DOT 5.1	270 °C (518 °F)	190 °C (374 °F)

Ingredients

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

Chemical	CAS no CAS registry number CAS Registry Numbersare unique numerical identifiers assigned by the "Chemical Abstracts Service" toevery chemical described in the...	Percent
Triethylene glycol Triethylene glycol Triethylene glycol, TEG, or triglycol is a colorless odorless viscous liquid with molecular formula HOCH2CH2OCH2CH2OCH2CH2OH. It is used as a plasticizer for vinyl. It is also used in air sanitizer products, such as "Oust" or "Clean and Pure." When aerosolized it acts as a disinfectant...		5-25
Tetraethylene glycol		5-25
Dibutoxy tetraglycol		10-50
Tetraethylene glycol diethyl ether Diethyl ether Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...		10-50
Propane Propane Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central... , 2-methoxy-1-(2-methoxy-1-methylethoxy)-		10-50

DOT 3 DOT 3 DOT 3 is one of several designations of automotive brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point....	Brake fluid Brake fluid Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure... s			DOT 5 DOT 5 DOT 5 is one of several designations of automotive hydraulic brake fluid, denoting a particular mixture of chemicals imparting specified ranges of boiling point.DOT 5 is a silicone-based brake fluid ....