Answer The U.S. Department of Transportation issues specifications for brake fluid. The three main types of brake fluid now available are DOT3, DOT4 and DOT5. The main difference is that DOT3 and DOT4 absorb water, while DOT5 doesn't. One of the important characteristics of brake fluid is its boiling point. Hydraulic systems rely on an incompressible fluid to transmit force.
Liquids are generally incompressible while gases are compressible.
This may partially or completely disable the brakes. As a DOT3 or DOT4 brake fluid absorbs water, its boiling point decreases. It can absorb water from the air, which is why you should avoid opening your car's brake fluid reservoir. For the same reason, you should always keep containers of brake fluid tightly sealed. DOT5 fluid does not absorb water.