As the other poster mentioned.. the reason the mower leaks down over time is likely a worn lift cyl / piston rings. Usually a 20$ NAA piston and 3$ worth of rubber oring and leather backup washer, plus about 5$ for top cover gasket, side cover gasket, and pto flange gasket plus some 4 hrs of sweat equity will correct it.
Now.. the reason you can't control the position ont he 3pt very well is that the 9n only has draft control, not position control.
You can either buy a 90$ bolt on position control linkage jig to make it have position control similar to an 8N.. or you can build/buy a set of limit chains like most 9n owners do. It a bracket that pins tot he toplink rocker and then has chain slots in it. You then have 2 sections of chain with an eyelet weled to one end. That eyelet fits over the implement pin and then the lift arm goes on.. do this on each side.. then lif tthe implement just slightly higher than you want it to set.. then slip the chains into the slots ont he bracket.. then relaxe the lift... Your implement then will stay at that height.. if you need to lift it to clear an obstacle.. go ahead... when you set it back down, the implement will bottom out on the chains and not go lower.
As for the loader.. Sounds like you have a trip bucket loader, and the hyds for the lift cyl are plumbed off the left side corner test port on the pump base plate.. kinda behind your right heel.
When you lift the 3pt handle, oil is normally sent to the lift cyl.. which lifts and when it reaches max height, internal linkage shuts the control valve off. However in this setup, you chain the upper lift arms down to the axle trumpets so that they cannot lift to full height.. then when you pull the 3pt handle up, the lift arms lift till they are stopped by the chain.. then oil is forced out the corner test port to power the lift cyl on the loader. This works like a SA hyds... when you lower the 3pt handle, oil is exhausted back into the sump. This is a pretty slick setup, because it still employes the OEM relief valve in the system.
The reason your mower and loader go up is because you havn't chained the 3pt upper lift arms down to prevent them from lifting up to max travel ( which shuts the oil off to the pump control valve ).
The gate valve in line to the laoder prevents heavy 3pt loads from making the laoder bucket come up.
Soundguy