During the air brake test, when should the automatic brake valve be cut out?

The correct response for when to cut out the automatic brake valve during the air brake test is after all locomotives' brakes apply. This action is essential for verifying that the brake system is functioning correctly under operational conditions. When the automatic brake valve is cut out only after all locomotives' brakes have fully applied, it allows for a clear assessment of whether each locomotive's brakes are operational and engaging as expected.

Cutting out the automatic brake valve at this point ensures that the test can confirm the integrity and response of the entire braking system before moving on to additional checks or releasing the independent brakes. It helps to maintain a thorough and methodical approach during the testing process, reinforcing safety by ensuring the brakes are in working order before reactivating any control mechanisms.