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I guess the tractor has position rather than draft control and that the 3ph lowers only when the control lever is down and stays up otherwise. I think you first need to determine if the pump is producing pressure of not. If there's a loader or something on the auxiliary hydraulic system that works, then there's pressure. If not, the diverter valve on the hydraulic manifold block can be turned to aux just long enough to hear the pressure relief valve open. Of course a pressure test would be better. The problem more likely is that something in the 3ph is stuck. I'm not aware that much can be done short of tearing down the control valve assembly, but maybe somebody knows some tricks. I'm more theory than experience, but I don't believe tearing down the control valve assembly is too bad a job. I does take a repair manual though (I have one for my own 1710). A stuck unload valve is a good candidate to cause the problem. The only trick that comes to mind is to try and create pressure surges in the high-pressure line that might loosen a stuck unload valve. If nothing is connected to the aux system, changing the diverter valve several times from aux to remote might do something. However, the pressure at the 3ph would only swing between zero and about 100 lbs, so I wouldn't hope for much. Of course the diverter valve should be left in aux only long enough to hear the relief valve open. Creating load shocks on the 3ph while it is in lift mode is that same idea. However, I don't think load shocks would do much due to the action of the check valve in the 3ph. In neutral or lift mode, shocks might help seat a stuck poppet valve, but that's not likely the problem if the 3ph stays up. Good luck with this. If by some chance the 3ph gets 'unstuck.' it'd be a good idea to change the hydraulic oil.
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