We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "Dave Liquorice"
saying something like:

Except that the OP has stated that the cisterm will completely empty if
left with the supply isolated for a day or two. Water level to high
causing an overflow wouldn't do that.

I've had a couple where the casting marks inside prevented the inner
seal from doing its job and they had to be ground down flush.