If the hydrualic system is normally open, meaning that oil flows freely from pressure to tank when control valves are not actuated, you could be seeing the result of the pressure compensating system at full pressure when you actuate a valve. It could also be a piece of dirt in the system causing the same issue, but I would suspect that the problem would remain, and not be heat related. I've spec'd and installed several industrial fluid power systems, and this sounds like the most logical scenario. I don't know if these are variable volume pumps, and therefore have a pressure compensating ring or plate in the pump or if they have a pressure compensating valve that dumps excess oil and maintains system pressure from a fixed displacement pump. I don't know the first thing about excavators, but I wonder if, after heating up, a pressure compensating valve spool is not shifting properly.
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