Any Zero-Turn Mower Experts?

Alright, just getting to this thing. Changed the hydro oil/filter, bled the system like Exmark says and it made zero difference. It's only when it's pulling a hill that it almost stops pulling on the left side. I'm thinking it has to be the pump or the wheel motor, but it still makes zero howling or growling (ran it at idle and I cannot hear any abnormal noise). Any other thoughts?