Leading-trailing brakes are adjusted _________.

Leading-trailing brakes are adjusted when applying the brakes in reverse. This is because this type of brake system utilizes a leading shoe that engages first in one direction and a trailing shoe that takes over in the opposite direction. When the vehicle is moving in reverse and the brakes are applied, the leading shoe has the opportunity to engage effectively, which is crucial for proper adjustment. This process ensures that both shoes are functioning optimally and evenly, allowing for balanced braking performance in both directions of travel.

The other options do not appropriately describe the adjustment process for leading-trailing brakes. Adjusting the brakes while applying them in forward may not provide the necessary mechanics for optimal adjustment due to the opposite engagement of the shoes. Manual adjustment does not include the dynamic elements necessary when the vehicle is in motion, and using the parking brake does not affect the adjustment of leading-trailing brakes during operation.