ICE coolant drain valve under the heat storage tank question

I changed the ICE coolant on our car yesterday, refilling by putting fluid back in through the drain valve under the heat storage tank. (Siphoned in from a container at a greater elevation.) I was unpleasantly surprised to find that that valve dripped out past the handle once it is opened, apparently worse on the refill than the drain. The outlet had a 5/16" tube on it, very tight fit, it was definitely coming out past the handle. Screwed the handle back in a bit and maybe that slowed the dripping, at the cost of also slowing the refill. However, once the valve was screwed shut there was no dripping at all. Total flow out the handle on refill, not sure exactly, maybe 20 ml?

What is the construction of that valve - is it supposed to seal tightly around the handle in the open position? Most likely the coolant is working its way out through the threads. If this can be fixed with a new O-ring or rubber washer (assuming the handle screws all the way out without having to pull the whole tank) it would be good to know for the next time I have to drain the ICE coolant loop. (Hopefully years from now.)

Thanks.