Tractor Repair: loader hydraulics, dry dirt, valve body

Question
Hello,
I have a tractor with a loader. with the tractor at idle when I move the control to move the boom up the boom goes down. If I rev the tractor and move the same lever the same direction ( UP position) the boom will move up. The boom is gutless will barley lift half a buicket of dry dirt.
Here is what I have done to troubleshoot:
rasied the boom/bucket several feet off the ground turned the tractor off and left it overnite. the bucket DOES not go down.
I have checked the pump pressure at the out put side of the loader valve 3500 psi.
so far what I have replaced:
HYD pump
many hyd lines
Loader valve.
all that is left is the cylinders for the boom portion. before I do that could you offer any other ideas?
Thanks
Mark

Answer
i really need to know what kind of tractor it is but does the bucket work ok if the tractor is reved up,if it doesnt then you could have air and if it is a ford then your valve body under your seat will need to be rebuilt,but email me back and let me know what kind of tractor it is please.