I know 'zero' about that tractor... That said... I'd start by looking at the control linkages and make sure that's all working freely. Lots of things can set up over winter. If that doesn't yield anything... Then I'd probably find the suction strainer for the charge pump assuming that it's a hydro machine... clean the strainer. Check for suction leaks, etc. Any charge pump should be a gear type and self priming provided the suction/sump is not obstructed.
This isn't one of those fancy joystick drive type machines with electronic controls for the hydro is it?