Wheel cylinders are essential components in a vehicle's braking system, specifically in drum brakes. They are hydraulic devices that help convert the brake fluid pressure into mechanical force, pushing the brake shoes against the inside of the drum to slow down or stop the vehicle. Typically located at each rear wheel, wheel cylinders consist of a cylindrical housing with two pistons inside. When the driver presses the brake pedal, brake fluid flows into the wheel cylinder, causing the pistons to move outward. This action forces the brake shoes against the drum, creating friction that slows the vehicle down.