Doosan DX60R Shop Manual page 95

Hydraulic system operating under conditions such as
repetitive cycling, heavy workloads, fluid circulating under
high-pressure, make it extremely critical that dust, grit or any
other contamination be kept out of the system. Observe fluid
and filter change maintenance interval recommendations
and always preclean any exterior surface of the system
before it is exposed to air. For example, the reservoir fill cap
and neck area, hoses that have to be disassembled, and the
covers and external surfaces of filter canisters should all be
cleaned before disassembly.
General Maintenance Procedures SP000016
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