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Information that is vital to the maintenance of
brake systems and brake fluid.

Know Your Boiling Point

The average driver uses his or her brakes approximately 75,000 times a year and expects them to function properly each and every time. With police vehicle operation, the number of brake applications can more than double that amount.(1) Within the braking system of all automobiles exists brake fluid, a highly critical component essential to brake operation. Brake fluid serves to produce the braking action as well as to protect and to lubricate brake system components. Every driver depends on brake fluid for stopping capability(2). The use of a high quality brake fluid is essential to assure the safety of the driver and passengers traveling in the automobile. As a public organization, we owe to the motoring public and our employees the safest environment possible.

How Do Brakes Work?

An automotive hydraulic braking system consists of a master cylinder, four wheel cylinders, metal tubing and flexible rubber hoses. This hydraulic system attaches to frictional components that are comprised of brake shoes and drums or brake pads and discs. The brake fluid is contained within the system assembly. When pressure is applied to the service foot brake, the master cylinder exerts a force of fluid. This force actuates the wheel cylinders to push the brake shoes against the brake drums or the pads against the discs, resulting in the braking action. Application of the brakes turns the energy of the vehicle into friction heat at the brakes.(3) This brings the brake temperatures to high levels. If the brake fluid has been contaminated by excessive usage of high heat the boiling point can greatly be reduced in a short period of time. A vehicle that has been in service for 18 months with 25,000 miles of service, could have the working temperature of the brake fluid reduced by as much as 25%. If the fluid's working temperature were reduced by 25%, the moisture content would be equivalent to 3%.(4)

Brake fluids classified as DOT3 and DOT4 are naturally hygroscopic: that is, they possess a strong tendency to absorb water. Air can contain varying amounts of moisture depending upon the relative humidity. This moisture can find its way into the brake system through flexible hoses, thus contaminating the fluid. Moisture contamination directly results in reduction in the brake fluid boiling point.

The frictional components in a braking system generate very significant amounts of heat, especially vehicles being operated in severe conditions like a police vehicle or service vehicle in mountainous regions. Vehicles used in operations such as these could have an increase of brake fluid temperatures of as much as 20%. This combination of higher heat and a reduced boiling point increase the possibility of fluid vaporization or vapor lock. Unlike brake fluid, vapors are highly compressible. When brake fluid becomes more compressible it loses its ability to transmit the necessary force to effectively stop an automobile. In the loss of brake force comes the loss of pedal response and conceivably, even loss of braking action. Vehicles equipped with Anti-lock Brake Systems (ABS) are much less likely to skid out of control, especially when the roadway is wet or slippery. The anti-lock system pumps the brakes automatically up to 15 times per second, thereby avoiding brake lock-up and skidding. The results are maximum braking and increased control. ABS systems, in some vehicles, send a sense of reduced resistance to the brake pedal. General Motors, in their Chevrolet Caprices, have added a booster kit to the vacuum booster, which increases the amount of resistance felt in the brake pedal. GM has also added larger wheel cylinders to the rear of the vehicle to create more braking power. (5) This also creates more heat in the brake components.

Conventional DOT3 brake fluids will absorb and average of 2% or more of water in the first year of usage. In that period, the boiling point can drop from 401 degrees to 250 degrees, a reduction of over 150 Fahrenheit degrees.(6) Brake fluid designated a DOT4 possesses an important characteristic, having low moisture activity or improved ability to resist boiling point drop. This boiling point drop becomes essential in assuring safe brake operation.

The following reflects both the minumum wet boiling point requirements for DOT3 and DOT4 brake fluid. (Wet boiling point refers to the resulting boiling point after moisture accumulation has taken place under controlled standards.) Taken from SAE.

Minimum wet
boiling point



A number of automobile design changes have occurred in recent years which directly or indirectly affect the braking system. The major influence on design changes has been for fuel economy. Drastically modifying the vehicle aerodynamics and design reduces turbulent air flow under the vehicle to lower the wind drag factor. Unfortunately this also results in a reduction in cooling air past the brake discs and drums. This potentially translates to significantly higher brake fluid temperatures in the wheel area. Improved aerodynamics and engine turbo charging have served to increase under hood temperatures. This will increase the temperatures in all brake components, from master cylinder to the rotors and drums. Front wheel drive vehicles and smaller lower profile tires equally increase brake fluid temperatures. (8)

The boiling point of new DOT 3 brake fluid is 401 degrees Fahrenheit. Contaminates such as moisture, dirt and corrosion greatly reduce the boiling point of brake fluid. High temperatures encountered in ABS disc brake applications require hydraulic fluid specifically formulated to withstand the higher temperatures than normally experienced in drum type brakes. Modern hydraulic brake fluids are designed for high boiling points because of the extended temperature ranges prevalent in disc brake service. Small amounts of contamination such as foreign material, vapor and water greatly decrease the boiling point. When the service temperature exceeds the reduced boiling point, contaminated brake fluid will vaporize and prevent positive braking on the subsequent application. Besides materially reducing the boiling point, moisture promotes rust in steel brake lines, sludge in cylinders and corrosion inside wheel cylinders and calipers.

Ford Motor Company
General Motors Corporation (8)
Friction Advisory Service
NAPA United
SAE (6)
Brake Design and Safety
Bosch Automotive Handbook
Motor Trend Magazine
NAFTA Fleet Bulletins
OTC Tools & Techniques
EIS Brake Parts (3)
Brake Tech/Talk volume (4)
Castrol North America (John Demko) (1,2,5,7)
Wagner Brake Products
WSP Fleet Bulletin
Revised Code of Washington
Code of Federated Regulations
Columbus Dispatch newspaper (4)

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