Odds are that if the vacuum breaker is acting up then it would be a good idea to replace the over pressure safety valve as well. The 15-20$ is cheap compared to fixing a hole in the roof (or in yourself). Some rebuilds I can get by with descaling the vac breaker, but I never recycle safety valves. Just is not worth it.

-Cecil

Believe me, every square millimeter of the machine has paid its dues and deserves to winter back home on the sunny coasts of Spain...

OK, so the vacuum breaker itself could be the problem? I haven't stuck a pressure gauge on the steam wand, but I assume (heh) that it's very high and assume (that word again) that the leaking vacuum breaker is the only thing keeping another plastic hose (or the boiler itself) from blowing apart as described in my first post.

As far as I can tell, with my limited experience in troubleshooting an HX machine, something isn't doing its job of telling the heating element when to shut off.

I will try to get a boiler pressure reading ASAP.

So using the cheapo, auto parts store pressure gauge I had on hand with a new pstat installed I got:
initial position: 24psi/1.65 bar
all the way CW: 35psi/2.4 bar
all the way CCW: 18psi/1.24 bar

At the max CCW position, I don't think the vacuum breaker was leaking anymore and heater cycle was much longer (minutes) rather than 30 seconds.

I can live with the boiler pressure around 1.2 bar, so maybe I'll call it good enough for now. I have a nicer set of gauges on order for future boiler and brew pressure measurement that I hope to eventually tee into the machine permanently.