Compressibility

This is not a specific DOT call-out, yet it is critically important to racers and enthusiasts. SAE J1705, Appendix A, A.2.2.8, states “Air Solubility – It has been reported that dimethyl polysiloxane fluid, which is a major part of silicone-based, low water-tolerant type brake fluids 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 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.“ The SAE standard continues, “A.2.2.8 – Compressibility – Silicone based brake fluids are more compressible than conventional brake fluids and the difference is magnified at higher temperatures.” Silicon-based brake fluids are described by DOT 5 (not to be confused with DOT 5.1), which should never be used in a racing vehicle due primarily to its compressibility.

Compressibility is largely ignored by those selling nearly all the brake fluids on the market -- and with good reason! It is better you aren’t told that their fluid could contain as much as 7% dissolved air!! Once they are tested specifically for compressibility, it is no surprise that certain "high end” racing fluids are known in the paddock for their poor pedal feel. A research report published by Union Carbide demonstrates a relationship between the compressibility of a brake fluid and its density (or specific gravity). The greater the density of a brake fluid the less compressible it is. As there is no DOT specification, within the scope of the Polyalkylene Glycol Ether-based fluids there can be a density difference range of over 200% between fluids! Since brake feel is so important to driver confidence and, ultimately, lap times, Torque RT700 has been successfully formulated to be the highest density brake fluid on the market today.

To demonstrate compressibility performance, you must have (1) temperature and (2) pressure along with the test result. There are a couple of other racing brake fluids claiming reduced compressibility however they either (1) do not offer you the data to support their claim; or (2) provide insufficient data i.e. they omit temperature or pressure.