Brake fluids mode of action
Brakes are activated by pressing the brake pedal, usually with the assistance of a vacuum servo system.
The pressure developed in the master cylinder is transmitted equally in all directions through the brake hydraulic pipes.
Pistons in brake slave cylinders of the disc brakes (or drum brakes) are moved by pressure in the fluid.
Thereafter pistons squeeze the brake pads (or shoes) against the discs (or drums). Friction between the pads (or shoes) and the discs (or drums) slows the discs and the connected wheels (or drums) down.