The wheel cylinder is a cylinder that contains pistons which use hydraulic force from the brake master cylinder to push the brake pads against the brake drum. The wheel cylinder is a critical element in the drum brake assembly. It contains fluid-activated pistons that push the brake shoes against the drums to slow the wheels. The wheel cylinder is also the source of many brake problems. If brake fluid leaks from the wheel cylinder, the vehicle could experience unreliable stopping, damage to new brake shoes, or partial brake system failure. A sticking wheel cylinder may cause brake drag, excessive brake pedal effort, and reduced braking efficiency.