When you push the brake pedal down, the vacuum control valve lets the engine draw a vacuum in the front section of the booster unit. The atmospheric pressure on the other side of the diaphragm provides significant additional braking force. Brake Fluid Brake fluid is a special liquid for use in hydraulic brake systems, which must meet highly exact performance specifications. It is designed to be impervious to wide temperature changes and to not suffer any significant changes in important physical characteristics such as compressibility over the operating temperature range. The fluid is designed to not boil, even when exposed to the extreme temperatures of the brakes.
Some newer cars use silicone brake fluids.
DOT 3 brake fluids and DOT 4 brake fluids can be mixed. It should be noted that brake fluid is highly corrosive to paint, and care should be used not to get it on your car's finish.