From what i remember on mine , although i never drain it , i just wrap a duvet around it in the cold .
I did once take it out , and found that the brass piece in question consists of 2 parts , 1 is the knurled bit that you unscrew in one hit , and the other i recall is a flat blade screwdriver screw inside the knurled piece , so either ptfe the knurled bit to the boiler or try to tighten the screw up , i think it is a pressure release valve , i mention it because i undid the screw and mine leaked until i nipped it up a bit with a screwdriver , maybe yours has worked loose ?