Thanks for the welcome.
I knew better than to do this, but things just slipped my mind. Now I've learned that my ancient brass manifold had a valve that was shot, so CO2 was wandering off. I put the whole business in the pool, and up came the bubbles.
I ordered a new manifold because it looks like upgrading the brass one to Duotight would be expensive and unproductive, and I feel like getting at least one new regulator. I have one that works, one I had to order new gauges for, and a third one that went swimming when my old keezer died and beer blew out of a hot keg. I have to have both beer gas and CO2 in order to have any kind of quality of life.
It looks like all three of my old regulators were beer-gas-capable. I have been trying to find out why anyone would buy a regulator that only works with CO2. Money, I guess.