If you order the independent lift kit on the 2520, I think it does come with an adjustment knob, similar to that of the 2305, that I've seen mounted on the left hand ROPS frame (left hand, meaning as if you were sitting on the tractor). Apparently this is supposed to "Lock" the independent lifting mechanism in the up position, as well as being the cutting height control. I have read numerous comments complaining that the cylinder does leak down when in the lock position, thereby not holding the deck firmly.
I have a 2520, I didn't like the idea of having to use 2 of the outlets from the SCV valve in order to power the mower lift cylinder, and the only way to get away from this is to add the JD kit that adds 2 more outlets at the SCV in order to keep all 4 lines to the loader connected, as well as the 2 lines required for the deck. With this set-up, you still need to change a control valve setting on the SCV when switching back and forth between loader and mower.
I didn't buy the independent lift for the mower, and use the mechanical system using the 3PTH to control my mower deck. This then leaves all 4 SCV outlets free for the loader, and saves the cost of the additional outlet kit, as well as the Independent lift kit.
When using the 3PTH as the mower control, JD does have a "deck lock up" kit, so you can lock up the deck, and still have the use of the 3 PTH, if you need it. The kit is kinda' expensive for what you get, (around $175) a couple of brackets and a pair of adjustable chains that you have to manually attach to the brackets, but it works, and I sure don't have the expertise to build one. A lot cheaper than the Independent lift, additional SCV outlets, and the labor to install it all.
For my needs, it was a lot cheaper to stay with the mechanical lift for the mower, and for the odd time I may want to use the 3PTH without having the deck also go up and down, I just attach the lock-up chains. Right now, I have my deck off, and front blade on, so the chains are holding up the deck lift arms. I have my ballast box attached to the 3PTH and have full control of the 3PTH, without the deck lifting arms also going up and down with the ballast box. And by the way, the chains don't leak down.
REV