What kind of tractor you got? You either have a leaking cylinder or a leaking valve. If your tractor has flow (speed) control for the 3 point try shutting it clear off with the 3-point raised and see if it will hold. If it does, it's your valve leaking. If it still settles, your cylinder needs new seals provided the flow control can completely stop the flow. If you don't have flow control you can't know for sure which is leaking by. If it's really consistent the way it settles, it's probably the cylinder. If sometimes it holds, and sometimes not, you probably have something stuck it the valve.