Brake fluid is an essential part of a hydraulic brake system. It is a type of a hydraulic liquid commonly found in automobiles. It also acts as lubricant for the rubber components and has corrosion inhibitors.
There are two types of brake fluid. One is glycol based which can be further subdivided by grade. It is used in vehicles with Anti Lock Brake System (ABS). While Silicone based are used in vehicles without ABS technology.
DOT 3 is glycol based. It has a minimum boiling point level of 401 degrees when dry and 284 degrees when wet. Although DOT 3 brake oil performs satisfactorily, many people prefer brake fluid with higher boiling points.
DOT 4 is also glycol based, with a minimum boiling point of 446 degrees when dry and 311 degrees when wet. The increased in the boiling point is caused by the addition of borate esters.
DOT 5.1 is developed to have a glycol based DOT 5 alternative. Thus, it has the same minimum boiling point as the DOT 5. It has lower viscosity levels than DOT 4 at 900mm2/s.
DOT 5 is silicone based and resists water absorption. This makes the boiling point remain relatively stable. It has a minimum boiling point of 500 degrees when dry and 356 degrees when wet.