There are two types of brake fluids:
- Denatured alcohol
The designation DOT3 and DOT4 are for denatured alcohol fluids, those tend to absorb water, reducing the fluid`s boiling point and subsequently require more frequent maintenance. DOT4 have a heigher boiling point than DOT3, ironically once the fluid (DOT4) does actually begin to absorb water its boiling point will typically fall off more rapidly than a typical DOT3.
DOT 5 is for silicone fluid, it does not absorb water. This means the boiling point remain relatively stable, but it also means that any water that goes get into the brake system tend to form water pockets, wich could cause brake corrosion. DOT5 does not dissolve paint finishes.
These two types of fluid should never be mixed in the some brake system, most motorcycle manufactures recommend DOT 4.