Is it odd that the bucket leaks down but not up.??
Also, I see no evidence of a leak at all. Can you have a leak that is not apparent to the naked eye.??
Yes, you can have internal leaks that you will never see. Such as, if the fluid are leaking past the piston seals in the cyl.
Gravity is the answer. Buckets or lift cyl can not lift with out power, but can drop under the weight of the system.
I should clarify this. If the piston seals are leaking, and if the geometry is such that the weight is pulling on the cyl shaft, then the shaft will extend. If the geometry is such that the weight is pushing on the shaft, from an extended position, then the shaft will retract.
There is danger in a system that leaks down as you indicated by someones bucket dumping a load of wood. What if a kid were playing around the tractor.
For safety reasons, FEL should not be left in the raised position.
As Jinman said, do the bucket cyl test and prove to your satisfaction, whether it is the cyl leaking.
Even new valves leak to some degree, and as they begin to wear, the drop down is to be expected, and you have to live with it or replace the leaking components. You could add pilot operated check valves in the cyl circuits, to compensate for the leaking valve, but it tends to slow things down.
You could take the cyl to a hyd shop and get them to do a load test for leakage. They should be able to pressurize the cyl and tell you if it is leaking. If you let them fix it, have them test if after new seals have been installed.
Always get an estimate for labor and ask is there a min time you have to pay. If you don't like their fee, then you can order the seal kit and do it yourself.
Do you have one cyl or two for curl?