It turns out my problem all along was in the grouphead. Wish I’d figured that out before I went on a full odyssey through every portion of Vesuvius’ internal plumbing!
The rubber seat on the brass plunger just below the cam (identified with the green arrow below)
![](https://i.imgur.com/WJ024Ge.png)
had worn in such a way that it was creating a seal and not opening:
![](https://i.imgur.com/EDPvKKJ.jpg)
Once I replaced that, I was up and running again – without any flow failures!