The loader raises (or lowers) partially and then stops

The loader "boom" leaks down slowly 

This is fairly common with quick attach loaders where quick couplers are used for the hydraulic hookup. Occasionally you will "snag" a quick coupler causing it to come partially uncoupled. Or, you didn't get it fully coupled when you put the loader back on the tractor. If either has happened, you will HAVE TO get the weight off the cylinders before you can reattach the couplers. There is hydraulic "back pressure" (from the weight of the loader) in the hoses preventing the coupler ball or "tip" from coming off its seat. DO NOT use a hammer on the ball or tip. It is a waste of time and you may seriously injure yourself. High pressure oil is nothing to play with. One way to get the loader down is to shut off the engine and just sit there working the loader control lever back and forth. Depending on how high it was to begin with, it should come down after repeated "strokes" of the control. If that doesn't work, you will have to come up with some arrangement to hold the boom up to relieve the pressure. If you are in doubt about how to do this, PLEASE get qualified help. With the weight off the loader boom, you should now be able to recouple the offending coupler.

Assuming that you don't have an oil leak anywhere, and the front of the tractor stays "up" when you raise it with the loader, this problem is caused by oil escaping around the "spool" in the control valve. A certain amount of this is normal because if the spool to valve body were a perfect seal, it would be so tight you couldn't move the handle! Don't let anyone tell you that you have "bad" cylinder packing causing this. Oil has the same "compressibility" as steel. So…if you want to shove that big cylinder rod down into the cylinder, the oil has to have somewhere to go. See the animation.