An automobiles brake system is a hydraulic system. To quote a verse from a trade school tech manual I have..."A hydraulic system is basically a system that uses a liquid to transmit motion or pressure from one point to another". The brake system uses a confined brake fluid to transfer brake pedal pressure and motion to each of the wheel brake assemblies.

Brake fluid is designed specifically for this hydraulic engagement. The caliper seals are also designed to work with this fluid. Any other fluid would get past the seals. Brake fluid is also designed to withstand boiling up to a given temp. Brakes get hot and the fluid can boil which will cause reduced braking capabilities. Dot 4's boiling point is a little higher than Dot 3's.

Brake fluid is hygroscopic which means it's a water magnet. This is why it's best to flush/bleed the system annually. Some say in the spring after winter's past. In some places there is no winter and spring is a very wet time. Bottom line - flush/bleed brakes when it's dry - lower humidity.

Simply put - motor oil is for engines, gear oil is for manual trannies/differentials, Dexron-III is for auto trannies, power steering fluid is for power steering units, coolant is for cooling systems and brake fluid is for brake systems.

Chemisty.