As Kenny says, if you have the independent lift kit, you use the 2 outlets on the SCV that would normally control the "curl" function of the loader. If you have the optional diverter valve, this gives you another 2 outlets, so you can leave all 6 hoses connected. You can control the hydraulic flow to either the 4 loader connections or the 2 lift connections by means of a control valve lever which is mounted on the lower inside fender, which you can reach from the tractor seat. The independent lift kit comes with an adjustment knob, where you can set the cut height, as well as a setting that will lock the deck in the up position
The mechanical lift uses the 3pth to lift the mower, therefore the 3pth moves up and down with the deck. JD has a lock-up kit, consisting of 2 chains and a couple of brackets, where you can mechanically hold up the deck, with the chains, which would then allow the 3pth to move independently, once the deck was "chained" in the up position. The cut height with this system is controlled by the position of the rockshaft lever.
I have a 2520, and the mechanical system using 3pth to lift and lower the deck. I also have the "chain" lock-up kit. Pretty much the only time I use the lock-up kit is when I have my blade on the front for snow plowing, and the lock-up kit holds up rear mower deck lift arms up and out of the way.
The only time I can see where you would use, or need the independent lift for the mower, is if you want to keep the deck on, and still have full independent control of an implement on the 3pth. I think in most cases, if I was using something on the 3pth, I wouldn't want the deck on at the same time.
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