there are all kinds of variations out there, but most can be classified as either "open center" or "closed center."
the definition has to do with the excess flow from the pump circuit during the time when no hydraulics are being used. open center lets the excess flow return to the reservoir. closed center has no return to the reservoir. with open center, the pump runs constantly at the same output whether you use the oil or not. closed center has built in valves to change the pump output so that when the oil deadheads the pump will reduce its output to almost nothing, and when you actuate a device the pump will adjust and give up to full output.
there are returns to the reservoir from hydraulic valves and the 3 pt lift cylinder, the lack of a return has only to do with when you are not using/activating the hitch or valves.
hard to keep it simple so hope this helps a little. there are better explanations out there and illustrations too.
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Today's Featured Article - The Fordson F Ignition System - by Anthony West. A fellow restorer contacted me earlier this year asking for some help and advice on a model F that he was restoring. He had over a period of months spent a fair amount of his hard earned cash on replacement parts for the old "trembler" ignition. Sadly though all his efforts seemed to be a waste of time and money as he still couldn''t get the temperamental old thing to run correctly!! If i said that this was a little frustrating for him that would be "conservative" in fact the problem had reduce
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