Hi Steve,
Well I was kinda thinking out loud this morning going out the door on my way to a job. I was pretty sure that you'd experienced the amount of energy to needed to keep a copper pipe dry whilst you braze it, it's enormous, red heat being of the order of 750°C is a huge amount of energy. In a small circuit, mostly filled with water, the pressure rise would be very fast, I would have thought that if it was circulating, the pressure would have gone up rapidly above 20 psi, I may well be wrong, (probably!) but I would have though you had a vaper lock!
Regards, Matthew