What component transfers force from the primary shoe to the secondary shoe?

The component that transfers force from the primary shoe to the secondary shoe in a drum brake system is indeed the adjusting screw. The primary shoe, upon application of brake pressure, moves outward and pushes against the adjusting screw. This action causes the adjusting screw to apply force to the secondary shoe, which then also moves outward to engage the brake drum.

The adjusting screw's primary function is to ensure the correct positioning of the shoes and maintain optimal spacing as the brake linings wear down. This makes it crucial for effective braking performance, as it helps to keep both shoes properly oriented in relation to each other and the brake drum.

The other components listed serve different functions: the wheel cylinder actuates the shoes by pushing them outward, the return spring helps pull the shoes back into their resting position when the brakes are released, and the brake drum is simply the surface against which the shoes press to create braking friction. Hence, while all these parts contribute to the braking system's overall functionality, the adjusting screw specifically transfers the necessary force between the primary and secondary shoes.