Final answer:
In a duo-servo brake system, the primary shoe is the leading shoe which first contacts the drum and has its braking power enhanced by the drum's rotation.
Step-by-step explanation:
The primary shoe on a duo-servo brake system is the leading shoe. In a duo-servo brake system, there are two shoes: the primary (leading) and the secondary (trailing). The primary shoe is the first to contact the drum when the brakes are applied, and due to its position, it receives the rotating force from the drum, enhancing its braking power. The force is then transferred to the secondary shoe, making it act as the leading shoe as well. It's important to understand that in this type of servo brake system, both shoes amplify each other's force due to the rotational direction of the drum.