What should not be maintained in the brake pipe when locomotives are detached?

Maintaining air pressure in the brake pipe is critical to the functioning of the braking system, especially when locomotives are coupled. However, when locomotives are detached, it's essential that the brake pipe does not retain air pressure. This is because the brake pipe’s pressure is designed to release and equalize when the train is separated, allowing the brakes of the detached locomotives to automatically apply and prevent unintended movement.

In the context of the options provided, air pressure is not supposed to be maintained in the brake pipe when locomotives are detached, as it's part of standard safety practice to ensure that brakes are applied immediately in the event of separation. This is vital for the safety of both the locomotives and the rolling stock.

The other options, such as hydraulic fluid, fuel levels, and brake fluid, are not directly related to the operations concerning the brake pipe during detachment, which is why maintaining air pressure in the brake pipe is specifically highlighted.