I'd try a couple of things. First, there should be a plastic cover on the top of the control valve that mounts on the back of the PTO unit. Slip that cover off and if there isn't a bunch of crud in there, you should be able to see where the linkage connects to the spool. Move the hand lever, and you should be able to see the spool move up and down. Sometimes the roll pin in there can get worn or broken, preventing the spool from moving far enough to engage the PTO.

The other thing I would do is, there are 2 steel hydraulic lines that go to the control valve. One goes back to the reservoir. You'll want the other. Disconnect the line, and hook a gauge up to it that goes to around 500 PSI. Start the tractor, and you should have somewhere close to 300 PSI on the gauge at all times. (I don't recall the exact PSI it is supposed to be, and my book isn't right here - sorry <!-- s:roll: --><img src="{SMILIES_PATH}/icon_rolleyes.gif" alt=":roll:" title="Rolling Eyes" /><!-- s:roll: --> ) If you're not getting pressure there, then it is something over in the set of valves on the left front side on the transmission. If you are getting the pressure, then it is either that control valve on the back of the PTO, or something got seriously put back together wrong on the inside of the PTO.