IF it is the bucket cylinders it could be the valve, or the seals in the cylinders.
To tell which it is, raise it about a foot off the ground and put a floor jack under the edge of the bucket. You are using this to relieve pressure in the hydraulics. Unhook the couplers going to the bucket and lower the jack but dont pull it out from under the bucket.
IF it still rotates downward like before, the issue is the cylinders. If the problem stopped, it is the valve. Now use the floor jack again to take pressure off the loader so you can re-attach the couplers