La Marzocco GS/3 Owner's Manual page 21

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Press the "continuous" button
into the coffee boiler and wait for the brewing phase to end.
Repeat twice. Now it is necessary to remove all of the air in
the coffee boiler to saturate the group. This is commonly
referred to as bleeding the group. To do this, remove the
group cover by rotating the cover counter clockwise.
Unscrew the bleed screw (approximately 1/2 turn) to release
all air from the coffee boiler (using the included 5mm hex
key wrench). When water leaks from under the screw head,
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tighten the screw. Note: It might be necessary to activate
the brewing process to force water into the boiler. To do
this, press the continuous key until water leaks from under
the screw head. Then tighten screw. Replace group cover
removed in previous step. The steam boiler will begin to fill
automatically and the reservoir will need to be refilled. Once
boiler is full, heating phase will begin and the expansion
valve will need to be adjusted immediately (see next step).
to allow water to enter
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