What is the reason for releasing the parking brakes before conducting a full brake application test?

Releasing the parking brakes before conducting a full brake application test is primarily important to avoid damage to slack adjusters and linkage. When the parking brakes are engaged, they hold the brake shoes against the drums or discs, which can put excessive stress on the braking components, particularly the slack adjusters and linkages.

During a full brake application test, the brakes are meant to be engaged fully and evenly to check for proper functioning and balance. If the parking brakes remain engaged, the additional pressure and movement can lead to malfunctions or even failure of these vital components. This check ensures that both the service brakes and the parking brakes are functioning correctly and independently, which is crucial for maintaining vehicle safety and reliability.

While the other options present considerations about braking performance and safety, they do not address the specific mechanical integrity implications associated with keeping parking brakes engaged during a brake application test.