I am about to install 3 new radiators with a single drop feed, and I know I need to install a drain cock at the lowest point. But do I install a drain cock only on the inlet section of pipe to the radiators, or on both the inlet and outlet sets of pipe? Not sure whether the water will run out of all the radiators using only 1 drain cock and I could have a small section of pipe which doesnt drain? What is the recommended way of doing it? Thanks in advance.