Why should firefighters open the bleeder valve or drain valve between the control valve and cap?

Opening the bleeder valve or drain valve between the control valve and the cap is a crucial step for ensuring safety and operational efficiency. By doing so, any trapped pressure in the system is effectively released. This is important because trapped pressure can create a hazardous situation; it may lead to unexpected force if the system is manipulated while still under pressure, which could result in injury to personnel or damage to equipment.

For example, when working with firefighting hoses or other high-pressure equipment, ensuring that all pressure is safely released before disconnecting or adjusting any components is vital. This practice helps to maintain a safe working environment and assures that firefighters can perform their tasks without the risk of sudden releases of pressure that could lead to accidents.

The other options focus on procedural order, water pressure maintenance, or valve threading issues, but they do not address the primary safety concern of releasing potentially dangerous trapped pressure in the system. Therefore, the focus on safely releasing trapped pressure is the main reason for this operational procedure.