Why must independent brakes be released before making any reductions in brake pipe pressure?

Releasing independent brakes before making any reductions in brake pipe pressure is essential to facilitate a complete air brake test. When the independent brakes are engaged, any pressure changes in the brake pipe may not accurately reflect the system's response or efficiency. By releasing the independent brakes, you allow for a full assessment of the braking system's performance under the conditions of pipe pressure reduction. This procedure helps ensure that the air brake system responds appropriately, confirming that all components function correctly and efficiently. Therefore, the focus is on the operational readiness of the entire brake system, which can only be achieved by ensuring that independent brakes are released prior to any brake pipe pressure changes.