I have a 34" Gravely Pro-G Walkbehind mower. It's a great machine and is belt driven, but it slightly veers to the left. I looked through the owners manual, but all it says about it is "Adjust Tracking." I've made sure tire pressure in both tires is equal and I tried tightening the brakes, but I only know how to tighten them on the controls, which does nothing for the tracking. How do I adjust the tracking so that it goes straight?