Tonight I made a chamber liner by silver soldering it at high temperature.
In my pressure test fixture it failed between 380 and 400 PSI.
So this explains my motor failures, but does not really give me a huge amount of comfort. It means I can never get the liner hot enough to anneal it and I am Dependant on the annealed condition of commercial water pipe for my strength.
We are going to test fire again the weekend.