I have a 75' 4000 that I am currently rebuilding the lift on. There are several things that might cause this (worn control valve and bushing, pump, bad o rings). However, I agree, It does sound like you do have a linkage problem. I will try to explain what is likely happening. Bolted to the back of the liftcap (the part that your seat is bolted too) there is a cylinder looking casting that your three point yoke is attached to. Inside that casting, there is a big spring. You can feel it by running your fingers underneath the casting as it is open. The draft works off of compression and tension of that spring from the implement being used (normally a plow). If the plow point runs too deep, the toplink mount becomes compressed, which pushes in on that spring. Inside the lift cap, there is a actuator arm that is connected to the spring and links up with the lift cylinder control valve steem. The actuator pushes in on the control valve steem, which causes the lift to raise the plow up enough to release the compression on the toplink. If I move the hudraulic lever all the way up and mash the draft lever down with my hand up comes the hog. It is like the draft lever is not going all the way down when you flip it down. The extra pressure that I apply with my hand is controlling the lift via the draft lever if I let off the draft lever the hog stops coming up and stays at whatever position it was at when I stop mashing the draft lever.The likely reason this is happening is because your quadrant lever (hydraulic lever as you call it) is not adjusted correctly to control the control valve steem. The control valve steem is about 6" long and it slides back and forth in a bushing. The valve has shoulders on it that either blocks the fluid or allows it to pass by and go into the lift cylinder. This is what raises your lift. It sounds like to me that when the quadrant lever is raised all the way up, the control valve steem is not pushed in far enough to properlly allign the valve's shoulders in the correct position to allow the fluid to go into the lift cylinder. The reason your lift works when you apply pressure to the draft lever is becasue the additional pressure you are appying causes the actuator arm to push the contol valve steem into the proper position to allow the fluid to pass, thus raising the lift.
If this is what is wrong, the good news is that it is a easy fix. Go by you a IT shop manual for your tractor. You can get these off the internet or at Tractor Supply. The manual tells you step by step how to adjust this. You will need to remove the lift cap to make the adjustment. To do this, you will need a engine hoist. The lift cap and clylinder weigh at least 100 pounds, and the cap sits on top of the pressure tube coming from the pump. You will need to be able to lift slowly to keep from damaging the tube or the lift cap. Also, the pressure releif valve is screwed into the end of the lift cylinder. You will need to lift the back of the lift cap up a little before the front so you are able to get the pressure relief valve out of the housing. This may be more than you want to tackle, but if you need any advice, e-mail me and I will be glad to help you any way I can. It would take a tractor mechanic around 2-3 hours labor to do this, plus a new gasket for the lift cap. I hope this helps you out.
