Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere.
silicone fluid should be used only to fill non-ABS systems that have not been previously filled with glycol based fluid. Any system that has used glycol based fluid will contain moisture; glycol fluid disperses the moisture evenly throughout the system and contains corrosion inhibitors.
Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal.
Non-hygroscopic fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life, although at the cost of potential phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used.
The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climate, particularly in Russia and Finland, as glycol based fluids have to be replaced at regular intervals.
Motul RBF 600 brake fluid (DOT 4)
Bogie is the man, and I should learn not to mess with him
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