''Question, if I plumb the unused port (lift spool on new valve) to the tank (frame return) does that mean it's gravity dropping?''
Yes, When you push the lever forward to lower the lift, the pressurized oil will be sent to the tank through the valve port that would supply pressure to the retract side of the lift cylinders, if you had double acting lift cylinders. At the same time the spool port for the extend (lift) side of the cylinders will be open to the tank (a non-pressurized circuit), allowing gravity to bring the lift down, retracting the cylinders.
''Current set up on old valve is one outlet ,with a tee in the loop, feeding both cylinders to lift the boom. Does that make sense?''
Yes, it makes sense. That 60 - 70 year old valve was designed to use with single acting cylinders such as the ones on your loader. Only one spool port is needed for the single acting cylinders. When you pull back on the lever the spool moves to align with a valve body port, that has pressurized oil in it, and allows pressurized oil to go to the extend side of the cylinders for raising the lift. Pushing the lever forward moves the same spool port to align with different internal valve body porting that is not pressurized and leads back to the tank, allowing gravity to push the oil in the extend side of the cylinders back to the tank.
If you do not connect the unused (retract) port of the valve to the tank, the pump will be supplying pressured oil to that plugged port. That will open the main relief valve, allowing oil to return to the tank via the relief circuit. The problem with that is it is unnecessarily loading the pump and can cause heating. That is why it is recommended to hose that valve port to the tank, it doesn't cause pressure to build inside the valve body.
The Summit P40 is available with or without a float spool. Did you purchase the P40 valve with the optional float spool or just the directional spool (no float position where the handle locks when pushed fully forward, until the operator pulls back on it). A valve without a float spool, I call a plain directional valve. Loader valves are directional valves but will have a float spool/position in my opinion. If you have a valve with float, it will work without porting the unused port back to the tank as long as you push the lever into float to lower the lift, as both sides of the spools are open to the tank in that position. Not ideal but will work.
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Today's Featured Article - Harvestin Hay: The Early Years (Part 2) - by Pat Browning. The summer of 1950 was the start of a new era in farming for our family. I was thirteen, and Kathy (my oldest sister) was seven. At this age, I believed tractor farming was the only way, hot stuff -- and given a chance I probably would have used the tractor, Dad's first, a 1936 Model "A" John Deere, to go bring in the cows! And I think Dad was ready for some automation too. And so it was that we acquired a good, used J. I. Case, wire tie hay baler. In addition to a person to drive th
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