Well first I guess I'll introduce myself. I have been operating a part time lawn service for four years now and I have just stepped into the wonderful world of hydrostatic transmissions which are awesome however I am now having an issue. I have done some research on this subject but have not been able to find what I'm looking for via search except someone who posted up with this problem on a new machine and never responded with his results. Anyway I am having a bit of trouble with what I would call the responsiveness of my hydrostatic pumps on my used 2006 Exmark Viking on extremely steep hills. If I ascend up a really steep hill, and I'm talking real steep like 30" grade or more and stop the machine then release the handle to move forward up the hill again the controls stick, and the mower moves very slowly. If I push the levers out with the back of my hand it engages the pumps fine and the mower will move at it's designated pace. There is never any shuttering or chatter of any sort, or any extra whine coming out of the hydrostatic pumps when my problem occurs. The handles just plain stick, and will not fully extend. On flat ground and small grades the mower will start, stop and turn with no issues, lever sticking or hesitation at all. I have also noticed the mower will back up any degree slope with absolutely no "sticking" of the handles at all. Does anyone have any suggestions as to what my problem could be? I would not really rule out air in the system as I have not touched the hydrostatic system since I bought the machine but please do not be too quick to mention wheel motors or pumps if you are not sure that is my issue. Thanks for the help.