I drink a lot of espresso. From my minimal knowledge of espresso machines, I understand that the water is heated into steam which creates pressure forcing the steam through the coffee grounds. After every batch of espresso I let the machine sit overnight unplugged. Presumably, the water gas mixture returns to room temperature. Every morning when I unseal the heating chamber pressure is released. Why? In the ideal gas law P = (k*m*T)/(V*μ*m_u) and T was returned to the starting value. Wouldn’t P also return to the starting value? Perhaps some other value in the equation is changing? There is less water which would mean m would be less at the end than the start but this would mean less pressure not more.