(quoted from post at 11:41:24 10/31/15) Take a look at Fig 6-15. The only thing you are doing when you adjust the draft sensitivity lever is moving the spool in fig 6-15 left to right, which has the effect of engaging the drain valve plunger either sooner or later. In the event that it might be adjusted too far in, you could be dumping fluid from the 3pt piston without knowing it while you're trying to lift. Simple exercise to test that, take the bolt off the sensitivity lever, and back the adjuster out another turn and try again.
I realize I'm going to sound like a broken record here, but I'm going to say this again. If you've changed the pump (with a presumably good replacement), then your problem is a missing or bad cooper sealing washer, or a linkage adjustment issue whereby you're dumping the fluid while you're trying to raise the lift. Since you said it holds when you get it raised, then I'm back to sealing washers. I drove myself nearly nuts trying to figure out why mine would not lift, until I found a missing washer.
I agree that inserting a pressure gauge in this system is not terribly practical the way the book describes, so I used a 3,000lb (operating pressure is less than 2,200psi) gauge and just plugged it into the remote outlet on the hydraulics. With the tractor running and your gauge inserted into one of the remotes, you can pull the aux lever and see if your pump is generating 2,000-2,100psi. If so, check the pump off the list of potential problems. If it is not, that does not mean the pump is bad, so don't make that conclusion yet. Next, take the hydraulic ports apart on the aux hydraulic head and check the washers. When you've confirmed everything is in place and sealing properly, re-test the pressure. If you have pressure now, then you've isolated your issue to the hydraulic head for the 3pt. Honestly, until you get to this step, you're guessing at what is wrong.
The reason I said to immediately check all of the copper washers is because it's very simple, doesn't cost $'s, and is most likely going to fix the problem. Obviously gets harder from there if they are all in place and doing their job.