My new mistakes:
1. The image above shows 2 seemingly identical .5 inch plastic valves. But the one on the right has tapered threads (about $10 from Adafruit). The left (cheaper) one did not. To get the brass fittings screwed on I had to carefully file down the end threads so I could get the adapters screwed on without butchering the plastic. Very time-consuming -- and irritating.
2. Also, why did I go cheap? For about $25 I could have a brass valve to go with the pair of $15 fittings. And the whole thing would likely have been more reliable.