Not sure my "fix" will work in your case: my lever was too loose. When I raised the lift I had to hold the lever in the up position to keep it from falling back down on its own.

I found nut #9 in the JD Parts List (posted in reply #3 above, by kennyd) was loose. This nut applies pressure to #11 (a set of "bellville" washers). I simply added a drop of Loctite and tightened the nut firmly. I haven't had any further issues with mine.

Sounds like your problem is more likely in the same area as Millalien described in #5, above.

Either way, I think you will need to remove your seat (complete with its support mechanism) and maybe the shroud covering the hydraulic valve. I removed both when I worked on mine, but afterword thought I could have probably managed to work through the shroud opening where the seat mounts. I didn't look at access to the area Millalien worked on, it very well may require shroud removal.

Good Luck!