The diagram in a previous post shows several relief valves; a1, a2, b1, b2, b3, b4, etc plus the relief ahead of the return filter. Getting a gauge on the return line as well as the high pressure line would also be helpful for diagnostics.Yes it occurs on all hydraulic functions but to varying degrees depending on the load - eg. raising arm drops RPM more than lowering arm, pushing the bucket drops RPM more than pulling the bucket.
Suction and return filters were both changed. Both were terribly clogged. Return was so clogged that it had pulled apart from the pressure. But replacing them hasn't made any difference.
I'm still trying to get hold of a pressure gauge.
Can anyone confirm that the steel block standing up at the end of the control block, which is connected to the return channel to the tank, is the relief/check valve? (see image). looks like the one in the manual.
How long between hydraulic filter changes? What's the OEM recommended change period. What was the material on the filters?
Is the oil cooler functioning? is it hotter at the inlet line than at the outlet line? Using a laser thermometer, what is the temperature of the cooler inlet flow and the outlet flow temperature?
I'm starting to think the that you may have a contaminated hydraulic system. Maybe a clogged cooler is causing the hydraulic oil to overheat and break down but it depends on what material you have on the old filters and the temperature you measure at the oil cooler.