Final answer:
The primary/leading shoe in a drum brake unit faces towards the front of the vehicle, utilizing the rotation of the wheels to enhance braking force.
Step-by-step explanation:
The primary/leading shoe in a drum brake unit faces towards the front of the vehicle. The leading shoe is designed to be self-actuating. When the brakes are applied, it receives the initial force and is pushed against the drum, making the front-facing side of the shoe more effective due to the rotation of the wheels. Conversely, the trailing shoe faces the rear and provides less braking force. In a moving vehicle, the drum rotates in the same direction as the wheel, and thus the leading shoe is pulled into the drum, enhancing the braking effect.