Pump/Wheel motor or????

I bough a used 2010 Husqvarna PZ6029FX with the Kawi 29 (25.5) with the knowledge that the left pump was week. The owner just put a new right pump on so I bought it and installed a new left pump. Flushed the system,new oil and filter. Now I wonder if it was the pump at all? On flat ground it does great! But on a hill, incline, anything to put it in a bind the left side stops pulling. Is there something I need to check or how do I check to see if its faulty pump or wheel motor. Other than the driving issue I love this machine!

Thanks!