How quickly? It's normal for it to creep down without any pressure in the system. That's why you are not supposed to work under the bucket without a dead man.
You will need to describe your problem in more detail before anyone can help you. Confirmed model of tractor and loader model will help. Also, what you are doing or trying to do versus what is actually happening. Is this a change from prior behavior or simply a results versus expectations issue? The more detail the better
Very common problem is the piston seals leaking internally (ya wouldn't see them leak outside), but it still could be the control valve.....Best to troubleshoot before repairs done
Since the purpose of the work ports on the control valve is to allow cyl movement, and to block any fluid from the cyl, you can have two possibility's. Could be cyl seals, or worn valve spools leaking , or a combination of both. .
Using a known good cyl, raise the lift arms and turn engine off. If the lift arms drop, the only logical place would be the control valve.