I took the brewhead 3-way solenoid valve off, and surprisingly I found the spring around that solenoid nucleus is a lot softer than I would expect. I know the pressure from brewing boiler could get about 12 bars when water expands, so how could this spring hold that much pressure? It is kind of stupid to ask, but I just really don't understand it. I also noticed there is another harder spring that holds the viton seal of solenoid nucleus, but it seems it has nothing to do with the functioning of solenoid valve.

When the solenoid valve is active (during extraction), the coil pushes the piston to the closed position using electromagnetic force. This force takes the place of the stiff spring you are referring to and resists the extraction pressure. When de-energized, the magnetic force is removed and the light spring returns the piston to the open position and lets the water flow out. This is a "normally open" (NO) type solenoid. A boiler fill type solenoid will work in reverse, opening when energized (well you can use NO, but that would degrade its service life and the programming for the controller is different, so dont just swap them).

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