[Marvin]

Both suggestions are excellent. Thank you both.

 

[Marvin]

So, before I made my post last night I drained the thing and noted only water, for the second time in a row. So I left it open with a bottle underneath to see of some other crap might eventually come out, went upstairs, and posted to y'all. Lo and behold, this morning I came out to find the usual root-beer-float-oh-so-tasty-looking crap had come out overnight. So I deduce that the issue was a blocked drain stopcock. I will give it a ream-out with a bamboo skewer and that ought to do the trick.

Thank you to all who helped out with comments. This continues to be a great forum. The signal-to-noise ratio here, while not perfect, is generally pretty good.

 

[Marvin]

A good thought Grimmy, but I am in Vancouver/White Rock, where frost is hardly ever even seen. But I think I will keep that in mind for future reference.