Brake fluid is a type of hydraulic fluid used in brake applications in automobiles and light trucks. It is used to transfer force under pressure from where it is created through hydraulic lines to the braking mechanism near the wheels. It works because liquids are not appreciably compressible. Braking applications produce a lot of heat so brake fluid must have a high boiling point to remain effective and must also not freeze under normal temperatures. These requirements eliminate most water-based solutions.

Brake fluid can come in a number of forms, standardized under the DOT (Department of Transportation) standard. DOT 2 is essentially castor oil; DOT 3, DOT 4, and DOT 5.1 are composed of various glycol-esters and ethers; and DOT 5 is silicon-based. Most cars produced in the US use DOT 3.

Glycol based fluids are 2 times less compressible than silicon type fluids, even when heated. Less compressibility of brake fluid will increase pedal feel. Glycols are hygroscopic and will absorb water from the atmosphere, reducing the boiling point of the fluid and degrading hydraulic efficiency. Changing fluid on a regular basis will greatly increase the performance of the brake system, but this is often not a concern in passenger cars. On the other hand, changing fluid at least every several years will preserve the life of brake system components (by removing accumulated water and other contaminants) and increase the overall reliability of the brake system. ¹ Brake fluid has many characters with dry and wet boiling points being the most popular to use when compared to other fluids. Department of Transportation (DOT) standards (for brake fluid) are from the Federal Motor Vehicle Safety Standards 116 (FMVSS116). When a brake fluid is “compliant” with DOT this means the fluid meets the specifications as stated by DOT. Another specification is the Society of Automotive Engineers (SAE). DOT 3, DOT 4 and DOT 5 brake fluids are said to be “compliant” with SAE J-17-3. J-1704 and J-1705 respectively. Any over-the-counter (OTC) brake fluid will have a statement on its label that says it meets or exceeds DOT 4 specifications. In other words the DOT “certification” allow consumers to compare the different fluids more evenly. We recommended that you use only DOT certified brake fluid. ²

All glycol based brake fluids are virtually incompressible and work well, as they are intended. However, problems begin to occur when brake fluid becomes overheated. Once the brake fluid begins to boil tinny gas bubbles are created. The problem… gas is compressible. Any time your brake fluid begins to bubble you will encounter a “spongy” pedal which leads to brake fade and eventually brake failure. It is highly unlikely that you will boil the brake fluid in your commuter car in and around town but that depends on your driving conditions. To play it safe it is best to use a high dry boil point brake fluid. GS610™ brake fluid it designed to perform in extreme braking condition on and off the track. Before changing to GS610™ brake fluid, we recommend that you completely purge your system by flushing the system and refilling with GS610™.

DOT 3 and DOT 4 brake fluids are very similar across the board with the exception of high temperature stability. Though a DOT 5 fluid has a superior boil point than standard DOT 3 and DOT 4 the compressibility factor of silicon brake fluid eliminates it good intention and is not recommended for extreme braking condition due to its compressibility.

All DOT 4 brake fluids have nearly identical chemistry; viscosity, compressibility and pH levels. But what separates the herd is higher temperature stability – Dry Boil Point (ERBP). Simply look at the GS610™ comparison chart against the several leading brands.

During extreme braking condition (hill driving, tow, constant stop-n-go traffic, high temperature claimants, recreational track events and professional racing) make sure your brake fluid has an extreme boil point. GS610™ has a maximum tested temperature of 626°F and is trusted by Andretti Green Racing, winners of the 2005 Indianapolis 500® with driver Dan Wheldon and 2005 Indy Racing League® Series Champions.

Dry Boiling Point - Equilibrium Reflux Boiling Point (ERBP). As tested by DOT - This is the temperature at which DOT compliant brake fluids begins to boil.

Wet Boiling Point - Wet Equilibrium Reflux Boiling Point (Wet ERBP). As tested by DOT - This is the temperature at which DOT compliant brake fluids begins to boil.

Viscosity - The higher the viscosity number the slower oil will flow. DOT 3 specifies viscosity testing at -40°C/-40°F at 1500 cSt with DOT 4 at -40°C/-40°F at 1800 cSt. Though DOT 3 brake fluid flows better than DOT 4 the boil points are not as high. Nevertheless, the likely hood of you experiencing -40°C/-40°F driving conditions is rare. A side note; GS610™ brake fluid flows at 1550 cSt. ³

Compressibility - Silicon based brake fluids are more compressible than conventional brake fluids and the difference is magnified at higher temperatures. 4 With only slight variations, DOT 4 brake fluids are virtually similar in compressibility at normal operating temperatures.

Air Solubility - It has been reported that dimethyl polysiloxane fluid, which is a major component of silicon based low water tolerant type brake fluids (SAE J1705), can typically contain dissolved air at a level of 16% ± 3% by volume at standard temperature and pressure. This compares with a typical level of 5% ± 2% by volume of dissolved air for glycol ether based SAE J1703 type fluids. An increase in brake pedal travel may be experienced under severe operating conditions, especially at higher altitudes and high temperature conditions.

The term “dissolved air” (air absorbed from the atmosphere) should not be confused with the term “entrapped” or “free air” since their effects on brake system performance can be entirely different. Air that has been absorbed from the atmosphere does not result in an increase in fluid or system volume, whereas entrapped air or free air does occupy system volume and can be easily compressed when force is applied to the system. 5

pH – A DOT specification that ranges between 7 - 11.5. The higher the pH level, the better protection you have against corrosion (over a longer period of time). Higher pH levels will reduce the brake fluid’s temperature stability. A pH level of 7 is a target number.

Sources:
¹ Wikipedia Encyclopedia Online Brake Fluid
² Federal Motor Vehicle Safety Standards 116 (FMVSS 116)
³ GS610™ Technical Data Sheet (TDS)
4 SAE J1705 Revised Proposed Draft June 2004 appendix A, A.2.2.9
5 SAE J1705 Revised Proposed Draft June 2004 appendix A, A.2.2.8

Visit www.familycar.com/brakes.html “A Short Course on Brakes” by Charles Ofria