I have a 1997 Grasshopper mower--model 616. Tractor SN#473258. My left hydostat is weak--has to be pushed all the way forward for it to work. Seems to get worse the longer I mow. I was told that maybe the valves have backed out, and need to be screwed back down. How exactly do I do this? Have removed the oil, and now am at a loss as where to go from here. Can you help me? Also, has anyone else had this problem, and what did you do to correct it? Have a repair manual, but it doesn't explain how to or where to find the valves, or how to fix them, or what to do when one hydostat doesn't work right. HELP!!