I got my tractor back, today, after 7 weeks and one day. It was the control valve assembly for the lift. It is the one with the knob under the seat. The part number is:007202923C91. It is large and takes a lot to get to it. The seat, rear lift and everything in between has to come off. The part is $900.I know this is an old post, did you ever find the problem. I have a 4530 with this same issue.