Hitachi ZW 120-6 Operator's Manual page 263

10. After replacing the filter, bleed air from the hydraulic
pump and check the oil level in the hydraulic oil tank.
(Refer to the descriptions for "Bleed air from the
hydraulic system" in item
If the machine is operated without bleeding the air
mixed in the hydraulic circuit, damage to the hydraulic
pump may result.
f
NOTE: Replace the element at the regular intervals to
maintain clean hydraulic oil and extend the service life of the
hydraulic components.
MAINTENANCE
.)
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