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Dennis has a good question.
OP: there is a TSB about replacing the fluid on earlier models, pre 1999 as I recall, that came with fluid that coagulated causing slow performance. It addresses fluid replacement. I don't have time to find it 4u today,sorry .
John
Firstly, I'm probably guilty of pursuading Oldelance to change the hydraulic fluid, as in my extreme heat environment in Bahrain, I've changed mine. I didn't "flush", just changed the fluid in the tank.
In my case, just removed the pump's bottom fittings and tilted the pump tipping the fluid out of the brass fitting in the top. After running the top a few times, a week later, I did it again.
Just make sure you have quality hydraulic fluid. Here's what I used. Came with the replacement hoses from British Parts.