The main components of brake fluid are glycol-ether (in DOT 3-4) and silicone-based (DOT 5). In the advantages of heat resistance, it has the disadvantage of absorbing moisture into it. The fluid with more moisture will cause the oil to have a lower boiling point. This causes the air is mix in the fluid. The air bubbles inside the brake pipeline cause difficult to brake when the brake fluid is hot.
Besides, the moisture causes wear and tear of the components inside the brake system, both metal and rubber, such as brake piston, brake valve and pipes, rubber parts in the cylinder.
Most cars today, whether manufactured in Europe, America or Asia, mainly use DOT4. If you want to know what grade your car is using, see the owner’s manual or see it on the brake fluid refill cap.