Just a point of clarification for others since I was a bit confused about the location of the vacuum breaker. The vacuum breaker is located at the end of that piece of pipe - just below where the silicon hose is attached. I disconnected the whole assembly at the boiler so as to not stress the tubing.
The vacuum breaker looked clean to me but it was definitely the culprit. No more hissing and sputtering after I wiped down the internals and re-assembled. Thanks again for the help!




