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Expansion valve
The 12 bar expansion valve
La Marzocco's manual says not to attempt repairs: any repair must be done by La Marzocco or an authorised centre, using original parts, and the warranty is void for components tampered with or repairs by unauthorised people. Treat this as authorised-centre work.
The expansion valve (B.2.041) is fixed at 12 bar. As the coffee boiler heats, the water in it expands, and the valve lets a little out so the group never goes above 12 bar. That is why the brew gauge sits near 12 bar during heat-up. It is not the brew pressure adjustment: that is the pump screw.
La Marzocco's checklist checks it yearly and replaces it every 4 years. One owner needed a new one after about 2 years, when it dripped all the time.