Final answer:
In a duo-servo drum brake system, the return springs link the primary and secondary shoes to pull them away from the drum surface after brake application.
Step-by-step explanation:
In a duo-servo drum brake system, the return springs link together the primary and secondary shoes. The reason for this is to ensure that after the brake is applied, and the hydraulic pressure is released, the springs pull the shoes away from the drum's surface, avoiding unnecessary friction and wear. These springs are critical for the brake system to release properly and reset for the next application.