La Marzocco GS/3 Owner's Manual page 10

7) Adjusting Water Pump Pressure.
The water pump is factory set at 9 bar
pressure. If it becomes necessary to
change the pressure please use the
following procedure:
1. Remove the left side cover assembly.
2. Locate the water pump adjustment
screw and loosen the lock nut.
10
3. Adjust the water pump pressure to
the desired measurement.
4. Rotate Clockwise to increase pressure
and counter clockwise to reduce
pressure.
NOTE: The water pump pressure should
be adjusted when the machine is
operating and coffee is present in the
portafilter.
Figure 7 - Water Pump pressure Adjustment