Brake Doctor – What are the pros and cons of fluids?
DOT3 and DOT4 are hydroscopic – they allow water to be absorbed into the fluid. This means that when heated up a fluid with water in it will boil more quickly (water boils at a lower temperature than brake fluid) which will lead to cavitation. DOT5 does not absorb water and will not strip paint work but as the water is not absorbed it simply sinks to the bottom of the system which is usually the caliper where operating temperatures are very high. Water in the caliper is bad as it can cause corrosion within the caliper although silicone fluids contain corrosion inhibitors and as water has a lower boiling point than fluid (100°C) there is more chance of boiling under relatively light braking. Glycol based fluids can strip paintwork where as silicone based do not. Silicone fluids also have a very long life which is why it is used in military vehicles which may be stored for years without use but be expected to be ready for action at a moments notice.
