The "drain" is the first picture. I have no idea what to call it or what it is for (I will refer to it as a drain for the rest of this reply because I don't know what else to call it). For reference the first picture is part of the pipe that is in bottom right of 2nd picture. What is this "drain" for? I have noticed that two other of the valves have similar "drains" (those appear to be working fine at the moment)I don't follow the drain comment.
What drain on the system?