2003 Yanmar Vio27-2 Left Driver motor slow/weak

Hello all, I have a 2003 Yanmar vio27-2 that the left driver motor is slow and weak when goes back. when the machine goes forward the driver works fine. If I try to back up toward an hill the left driven won't turn and I can hear a pressure sound from the body valve. If I pull both handle look like the right driver helps to move the machine, but if I pull only the left one the machine won't move. I did replace the hydraulic fluid and the filter already, but no change. I also loose the track, but no difference.

Hope someone can give me some ideas.

Thanks