The question is: Can you remove part of a system that is full of water without a leak? the answer is: 1. If you freeze all the water first so it is no longer a liquid. 2. If you create a pumping action to make a negative pressure in the system so atmospheric pressure pulls the water away from the radiator. Other answers would not be as serious. Water is a very difficult substance to contain without valves so draining it is a requirement.