After connecting the manifold gauge assembly, what position should the service valves be when topping off a system with vapor?

When topping off a system with vapor, the correct position for the service valves is the mid-position. In this position, the valves are neither fully closed nor fully open, allowing for some control over the flow of refrigerant from the manifold gauge into the system.

By being in the mid-position, you can safely introduce vapor into the system while preventing any liquid refrigerant from potentially entering, which could create operational issues or damage to the compressor. This adjusted flow helps to ensure that the refrigerant enters the system gradually and correctly, allowing for the system to stabilize properly.

Other positions, such as closed or bypass, do not facilitate the controlled introduction of vapor. The closed position would prevent refrigerant from entering the system altogether, while a bypass position is typically not applicable for routine servicing in this context. Therefore, the mid-position is the best choice to achieve the desired effect of adding vapor safely and effectively during the topping-off process.