The usage of spring is brewing groups is starting to be less common then it used to be. In the past when most espresso machines where mechanically controlled they relied in a lot of functions on the functioning of compression springs.
The most famous spring operated espresso machine brewing group is the Faema E61 brewing group. But also the E61 style of brewing groups used on various espresso machine brands such as Vibiemme(VBM), Rocket, Bezzera, La Pavoni, Wega, ECM Heidelberg, La Scala. These compression springs make sure that the valves get automatically closed when they need to be closed.
So they are responsible when the water inlet valve is opened with the came shaft, that the drain valve in the bottom section will be closed so that the water can flow out of the brewing group true the portafilter. When the lever is lowered the inlet valve is closed with a spring.
