danielpugh interesting, I assumed you had a memory metal vacuum breaker like on the alpha, but I notice it’s a standard one. So check the vacuum breaker and the safety valve for leaks. All the boiler fittings have the Sigilant sealant sweatmarks so they are likely to be fine.
With the machine on, use a wooden or plastic spoon handle and press down the vacuum breaker pin and blast some steam through for 4 or 5 seconds. That might steam clean it and may well seal fine with no more hissing.
P.S. Whatever you do, don’t remove the vacuum breaker circlip, only regret lies in that action.
