Brake fluid serves as vital purpose of transferring foot pressure to the brakes or hydraulic clutch system. Hydraulic system rely on an incompressible fluid to transmit force. Liquids are generally incompressible while gasses are compressible. If break fluids boils and becomes gas, it will lose most of its ability to transmit force. It can also absorb moisture from the air which can cause the fluid to degrade and not work properly. Hence, most mechanics suggest changing the fluid for brake every one to two years. Two main concerns when it comes to brake fluid are:1.What should the boiling point be? 2.Which fluids are compatible?
The two chief types of brake fluids are glycol-based, which can be further divided by grade(DOT3 ,DOT4),and silicon-based fluids(DOT5).Glycol-based fluids are basically used in vehicles with ABS(Anti-Lock Braking System) while silicon-based ones work only in vehicles with ABS. If a vehicle without ABS ever uses glycol-based brake fluid, we cannot switch to a silicon-based ones as small amounts of glycol will remain behind and chemically compromise the integrity of silicone.
DOT3 is the most common type of brake fluid used in domestic cars and trucks .According to the Society of Automotive Engineers(SAE),DOT3 can absorb 2 percent of its volume in water every year. Over enough time excessive moisture will cause corrosion in the brake system which can lead to issues like vapour-lock or spongy pedal.
DOT4 is formulated for use by all vehicles,it has a higher boiling point than DOT3 and it does not absorb moisture as fast.DOT3 and DOT4 are interchangeable, however it’s best to avoid adding DOT3 fluid to the system that already uses DOT4. It’s preferred for street and high performance applications.
DOT5(or synthetic brake fluid) is silicon-based and does not absorb moisture. Many street rodders used it as it’s non-corrosive to paint or other brake components. DOT5 fluids usually have a violet tint in colour as it can not be mixed with any other type of brake fluid.
It is a non-silicon-based polyglycol that usually has highest rated boiling point(over 500 degrees) and is best recommended for severe duty and high performance applications. Unlike DOT5, DOT5.1 can be mixed with DOT3 or DOT4.