Why is liquid used in the tubing bike brakes?

Bicycle brakes use a hydraulic system to transfer the force from the brake lever to the brake pads, which press against the wheel rim to slow down or stop the bike. The hydraulic system uses a liquid called brake fluid, which is typically mineral oil or DOT 3 or DOT 4 brake fluid. Brake fluid is used in bike brakes because it is:

Incompressible: Brake fluid must be able to transmit force without being compressed, so that the force applied at the brake lever is directly translated to the brake pads.

Low viscosity: Brake fluid must flow easily through the narrow passages of the brake system, so that the brakes can be applied quickly and smoothly.

High boiling point: Brake fluid must be able to withstand the high temperatures generated by braking, so that it does not vaporize and cause the brakes to fail.

Non-corrosive: Brake fluid must not corrode the metal components of the brake system, such as the brake calipers and brake lines.

Compatibility with rubber seals: Brake fluid must be compatible with the rubber seals used in the brake system, so that the seals do not swell or deteriorate.