Weak travel ...ideas on where to start ?

Hello, Had a question on where to start on what seems to be weak travel motors. Just purchased a hitachi ex 300 and have put a few hours on it testing the machine out. Its getting ready to have the pumps pulled to replace a seal on the bellhousing side. The gear oil is leaking out. I wanted to make sure that it was the gear seals and not the pump seals so I went to the dirt pit to dig an hour or so. The machine tracks straight and pulls hills just fine but seems when using one track or the other by itself and its on uneven surface or its hung up on rocks it wont rotate. When I get in this postion I can engage them both and it comes right out. The machine seems to have plenty of hydraulic power digging so not sure where to start. If its the pumps I thought since I will have them out I can look into what they might need and freshen them up myself. Thanks for any help.