Question

why is liquid (the hydraulic fluid) used to operate the breaks in car

Answer 1

because it is highly lubricating liquids

Answer 2

Brake fluid is a type of hydraulic fluid used inhydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplifybraking force. It works because liquids are not appreciably compressible.