I have about 900 hours on my tractor. I mow about four acres and it is very rough due to night crawlers. The hydrostatic pedal no longer returns to neutral from both the forward and reverse positions. I have: replaced the ball bearing that centers the valve shaft -- replaced the centering spring -- made sure the linkage is lubricated and free moving -- adjusted the centering of the hydrostatic valve. I am beginning to believe the problem is wear on the valve shaft. Has anyone had this problem and did they have to totally disassemble the hydrostatic transmission to repair it?