Mower will not move forward or backward at all

I have a Powr - Kraft 18/42. The mower was stuck in forward and would move unless I moved the hydrostatic relief lever. It turns out that the linkage was rusted. I sprayed it down with WD-40 and got it freed up. That is why I could not move it to neutral or reverse. I have the linkage moving now but it seems as if the hydrostatic relief is now stuck so when I put it into forward or reverse it does not move at all. What do I need to do in order to repair this problem. I have checked the drive belt and it is in place, although a little loose. It was moving forward days ago. I have not done anything other than spray with WD -40

Any help would be greatly appreciated.

Thanks!