Why do you need to remove the radiator cap when draining coolant?

Removing the radiator cap when draining coolant serves the critical function of preventing a vacuum from forming within the cooling system. When the coolant is drained, the system can create a vacuum if there is no way for air to replace the volume of the coolant being removed. This vacuum can hinder or completely stop the proper flow of remaining coolant out of the radiator and engine, leading to incomplete draining. Additionally, the vacuum can impede the filling of the system with new coolant later. By removing the cap, air can freely enter the system, facilitating proper drainage and ensuring that maintenance tasks can be performed effectively.

While the other options touch upon aspects of coolant management, they do not address the primary concern of preventing a vacuum, which is essential for optimal drainage and system functionality.