La Marzocco GS/3 Owner's Manual page 9

6) Adjusting the Expansion Valve.
The expansion valve is a component
that limits the maximum pressure in the
coffee boiler. Heating the coffee boiler
causes the water within to expand. Since
the coffee boiler is completely saturated,
the expanding water causes an increase
in pressure within the boiler. Without a
safety device the increase in pressure
could cause a rupture in the boiler.
The pressure in the coffee boiler should
never exceed 12 bar. The valve is hot, so,
using adequate protection, rotate the
expansion valve clockwise to increase
pressure. To decrease pressure rotate
the expansion valve counter clockwise
(See the following diagram).
WARNING
THE EXPANSION VALVE CAN DISCHARGE
WATER AS HOT AS 200°F / 93°C.
ADEQUATE PROTECTION FOR HANDLING
THIS COMPONENT IS NEEDED BEFORE
ATTEMPTING TO ADjUST.
Figure 6 - Expansion Valve Adjustment
9