Expansion valves are found in varying locations on the right-hand side of the steam tank depending on the age of your machine. The expansion valve limits the pressure in the brew system by releasing a few drops of water when the pressure reaches 11.0 to 12.0 bar in the brew system. There is no air compressed in the brew system so the water (as it is heated) will expand until it is released. If the expansion valve is worn out it will bleed off pressure prematurely and limit the brew pressure.
TIP: It is normal to read 11.0 to 12.0 bar on the brew pressure gauge when the machine is at idle.
For example, we have set the expansion valve to a point that is too low of a pressure so that it leaks while we brew an espresso shot. Although the brew pressure gauge shows 9.0 bar, we are actively losing water from the expansion valve. When we turn off the group from brewing, the pressure drops off and does not achieve a maintained pressure above 9.0 bar. Therefore, we need to adjust the expansion valve to achieve an idle pressure above 9.0 bar.
Using a small crescent wrench, turn the expansion valve base clockwise to increase spring pressure. We will test our adjustment by running the group again and cycling it off and watching the brew pressure gauge needle climb. It takes a little bit of effort to get it just right, but your adjustment is complete when the brew gauge measures 11.0 to 12.0 bar when the system is heating but not being used.