I have a used GDSS w/ 18 hp Kohler. The hydraulics are all over the place. I have adjusted the levers for neutral. The tank is full. The Filter is new. The system has been bled. At full throttle, running along in a straight line is next to impossible. All of a sudden one side or the other will want to grab and throw you off course. There is no such thing as a straight line with this machine. I have to turn the throttle down (there goes blade speed) and constantly play with the sticks to TRY and stay straight. This is my 1st hydro. Should I replace the fluid? Some of it is old. If I change the filter, again, replace the fluid and re-bleed the system, do you think my chances are pretty good at getting somewhere with this? I just hope that I am not looking at new hydro pumps. That can be quite expensive, so I am told. I would appreciate any and all info that anyone is willing to offer. I have the owner's manual and have read it front to back. Thanks in advance to any info that you are willing to share.