Glad you got some quality time w/ your pump. But, as you discovered, that's not what is causing the lift arms to drop.
If the arms drop w/ the tractor off or the clutch depressed, chances are you have a worn lift cylinder or piston (or both) or a leaky pressure relief valve. Take the inspection plate w/ the dipstick off. Lift a heavy implement & look inside w/ the tractor running & the PTO engaged to see if you notice oil running from the top cover. If oil is running steadily from the top cover the lift cylinder needs to be rebuilt. If no oil is running down from the top cover, look at the rear of the pump in the oil with the load still on it and see if you notice turbulence in the bottom. (Some movement of oil will be normal with the PTO shaft turning). You could have a weak pressure relief valve which will be detected by turbulence in the oil. If you see turbulence behind the pump, raise the implement as much as possible, TURN THE ENGINE OFF then reach in and feel the end of the pressure relief valve. If it is leaking you might feel the pressure push your finger off of the end. All pressure relief valves that fail the finger test are bad & should be replaced. But, not all pressure relief valves that pass the finger test are good because some can have slow leaks. To check for a slow leak, lift a heavy implement, turn the tractor off & put a ½ inch clear plastic hose on the end of the valve & stick the other end of the hose through the inspection port. With pressure in the pump, a bad valve will likely show some fluid in the hose. Or, you could just remove the valve & replace it & not worry about that being the problem!