Can anybody tell me what brake fluid is, I know we use it in our hydraulic brake system but it's not really hydraulic fluids
Now-a-days brake fluid is generally made of glycol-ether, which is a non-petroleum based fluid and therefore is less likely to damage rubber seals. Why not use water? Brake systems get hot and where water will boil at 100c degrees, glycol-ether boils at closer to 140c degrees, but it also helps reduce corrosion in the system and maintains its viscosity better with temperature changes than water.
Glycol-ether also has a far lower freezing point than water. Not so important in Thailand, but I can imagine some countries where this is very important.
The difference in DOT ratings refer to the boiling point and also what the brake fluid is constructed from – higher DOT rated fluids that are used in racing are often more corrosive than road fluid and shouldn't be left in the system for a long time.
Why do you need to regularly change your brake fluid? Brake fluid is hygroscopic – so it attracts water from the air and should water enter your brake system (it will naturally) it affects the performance of the brakes and can lead to a 'spongy' lever feel as the water compresses differently to the brake fluid.