of compressibility, even with varying temperatures to accommodate different environmental conditions. This is important to ensure consistent brake pedal feel. As compressibility increases, more brake pedal travel is necessary for the same amount of brake caliper piston force. Service and maintenance Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life. Ideally, 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 (DOT 3/4/5.1) will contain moisture; glycol fluid disperses the moisture 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. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climates, particularly in Russia and Finland. Brake fluids with different DOT ratings can not always be mixed. DOT 5 should not be mixed with any of the others as mixing of glycol with silicone fluid may cause corrosion because of trapped moisture. DOT 2 should not be mixed with any of the others. DOT 3, DOT 4, and DOT 5.1 are all based on glycol esters and can be mixed, although it is preferable to completely replace existing fluids with fresh to obtain the specified performance. Brake fluid is toxic and can damage painted surfaces. Components Castor oil-based (pre-DOT, DOT 2) Castor oil Alcohol, usually butanol (red / crimson fluid) or ethanol (yellow fluid) (methanol) Glycol-based (DOT 3, 4, 5.1) Alkyl ester Aliphatic amine Diethylene glycol Diethylene glycol monoethyl ether Diethylene glycol monomethyl ether Dimethyl dipropylene glycol Polyethylene glycol monobutyl ether Polyethylene glycol monomethyl ether Polyethylene oxide Triethylene glycol monobutyl ether Triethylene glycol monoethyl ether Triethylene glycol monomethyl ether Silicone-based (DOT 5) Di-2-ethylhexyl sebacate Dimethyl polysiloxane Tributyl phosphate See also Hydropneumatic Pascal's law Brake fluid pressure sensor References External links Car care council How Stuff Works: What are the different types of brake fluid?