madrooster, closed and open center:
JD has so called "load sensing" hydraulics on their bigger tractors (i suppose your 650 has a cheap and simple gear pump, because its a machine in a different price class than a 7030 series, and you say its a Yanmar)
Open center (continous flow) pumps need open center valves: If no spool valve is operated, the oil flows through the open center spool valves back to the reservoir, with minimal restriction. If the spool valve would be closed center, the oil has nowhere to go, and is blown off by the pressure relief valve at max pressure, with lots of energy loss and heat buildup.
Pumping oil around in the system, allways has some energy loss, but pumps are 1/5 of the price of a variable pump with load sensing controller.
Load sensing hydraulics need closed center spool valves: A load sensing system has a variable flow pump, which is controlled automatically. when no oil is required, the pump builds up to max. system pressure because the oil cant go anywhere when no spool valve is operated. As soon as the max pressure is reached, the load sensing thingy, swings the pump back in flow, untill its in a neutral position: it does turn, but doesnt deliver oil.
As soon as the spool valve is operated, the pressure has a way out of the system, and the load sensing device swings the variable pump back in work mode.
A load sensing system is more durable (the plungers of the pump dont cause mechanical friction when rotating in null stroke mode) and they build up less heat, because they dont circulate oil flow through valves, hoses and filters when its not required.
Regarding the size of a JD 650, i'm 99% sure that JD uses a gear pump in this tractor, because its 80% cheaper. That means you require an open center hydraulic spool valve.
To be 100% sure, ask a JD guru about the type of pump used on your machine.
About plumbing in extra spool valves:
Make sure you NEVER use T pieces in an open center system. In that case, the oil flow will run where it meets least friction, which is via the open center of the OTHER spool valve back to the tank.
You need it to enter 1 port, exit the other, into the 2nd spool valve. (connected in series, not parallel)
Make sure the spool valve has two exits for the oil flow, 1 going on to the 2nd spool valve, 1 for return flow. My brother once plumbed in a frontloader that wouldnt drop, because another spool valve pressurised the return line...
