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.
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.