Brake Fluid Solubility

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A.2.2.8 AIR SOLUBILITY—It has been   reported that dimethyl polysiloxane fluid, which is a major component of   silicone 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 (DOT 3 and DOT 4) type brake 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.

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A.2.2.9 COMPRESSIBILITY—Silicone   based brake fluids are more compressible than conventional brake fluids and the   difference is magnified at higher temperatures. The compressibility of SBBFs (Silicone-based brake fluid) may   be calculated at any combination of temperature and pressure [see J. A. Tichy   and W. O. Winer, “A Correlation of Bulk Moduli and P-V-T Data for Silicone   Fluids at Pressure up to 500,000 PSIF,” ASLE Transactions 11,338 – 344   (1968).]

The   effect of brake system performance, specifically pedal travel, may also be   calculated by the following equation:

MC Piston Travel = Compressibility x Fluid Volume                                                   MCPiston Area

Pedal Travel= MCPiston Travel x   Pedal Ratio

Development and improvement of SBBF   is continuing with special attention being given to the requirements of   individual manufacturers. The performance properties of SBBF will be more fully   defined through field tests and laboratory test data demonstrating satisfactory   SBBF functional capability in all types of motor vehicle braking systems. Other   low water tolerant type brake fluids, although not presently available, may not   necessarily exhibit similar physical property characteristics to the SBBFs and   may require separate recommended practices.