Didn't see your post until just now. I have a JD790 and added the rear hydraulic kit so that I can run T&T'. If you use the John Deere Kit (I beleive the 770 and 790 are the same in this department) it requires that you you use an open center valve and does not mention any dump to the slump. Since the rear mount kit replaces the pressure line between the SCV and the rock shaft housing you need pressure to pass through the valve and back to the rock shaft housing or your 3 point won't work. In my T&T application everything works as should as long as I do not have two valves in operation at the same time, and I do not have a return to slump. If you have a power beyond valve setup, you would need the return to slump. From what I found out in my research, as long as you are not depending on gravity to return a cylinder, you can plumb through a non-power beyond valve. If you have pressure returning the cylinder, the fluid will force its way though the rockshaft and back to the slump. Since you are looking at log splitter, you need power on both sides on the cylinder so a regular open center valve should work.
