Small leak found, in a probably trouble spot.
This is a Milwaukee MA957 regulator. The very small leak I found was where the cap screws down on the bubble counter. I often need to remove this to add more water and the interface is simply brass on the plastic rim of the clear part. HArd to tighten properly anyway because I don't want to strip the threads so it's likely I wasn't doing it enough. Smeared a small amount of Teflon pipe dope on the threads and now seems sound. Also turned the desired pH up to 7.5.
Pro tip: "Bubble Fun" kids' bubble soap would appear to have a shelf life, even if it's sealed and been sitting on the shelf for a couple years. After not finding any leaks again, I decided I needed to blow some bubbles myself and it was completely dead. No bubbles. Who'd a known?