If it is sucking air then it would likely be evident in the fluid in the resevour, it would be a golden color with lots of small bubbles, very noticible. If all of the hoses/fittings are In good condition then the problem is inside the pump, either seals or your thrust bearing is on its way out and caused one or more of the pistons in the cylinder block to seize up
Either way youll have to remove the pump from the mower for replacement or repair. Get a parts diagram from this link below, take the pump apart and inspect for damage. In my experience the majority of the time the thrust bearing is the culprit. If you catch it early enough the pump can be repaired.
More than likely the pump is a hydro-gear p-series...
When disassembling the pump make sure you mark the orientation of the charge pump on the pump itself. If it is assembled 180* incorrectly the hydro pump will not work
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