I remember warning about finding out which valve it is. https://coffeetime.freeflarum.com/d/1018-think-my-anti-vacuum-valve-on-my-lelit-bianca-is-bad/3 You must open the top and absolutely identify where the leak is…pop up a photo with where you believe it is if you want…and I can tell you what the component is.
Absolutely watch the video below where all components are explained. It’s for the first version but the vac breaker and safety valve are the same. skip to 3m 25 if you only want to see those, but I think it’s helpful if you grab a coffee and watch it all.
Replacement is easy, it will undo, but there might be thread locker. Use an open ended spanner on the large brass part of the valve and tappy tappy with a larger spanner to shock it off…don’t torque the thing off, or you will most likely break something
You need to be careful of the type of safety valve, not only the pressure rating (which should be the same or very similar), but how large it is…so it can sit sideways on the top of the boiler! Most large valves won’t fit…but the one BB linked to looks like it should and the pressure will be fine…you don’t want them blowing to early…3 bar is higher than stock, but will be fine.
Use a 3 way brass connector…get one from ebay if BB don’t have any…then you won’t ever have any worries again.