Ricoh D124 Detailed Descriptions Manual page 106

 When the negative pressure (suction) becomes strong enough, it will pull the
actuator on the end of the punger into the gap of the ink end sensor. This will
signal that the ink cartridge has run out of ink and trigger the ink end alert for the
cartridge.
Here is a side view of the ink pump mechanism.
 The ink pump motor (1) drives the ink pump cam (2) counter-clockwise and
compresses the ink supply tube (3).
 The pressure on the tube (4) drives the ink bolus (5) ahead of the cam and creates
enough suction to pull ink through the tube behind the pressure point.
 This action of the cam alternately applying and releasing pressure against the side
of the tube drives the ink out of the pump and into the tube (6) to the carriage unit
and print head sub tank above.
D124 100 Detailed Descriptions