L555 skid steer right steer issue

I brought a l555 skid steer and have a issue with it you go to move the right handle forward a little bit the machine will start to move forward but if you move the handle all the way forward machine will stop and won’t move like theirs not enough hydraulic fluid in it I have check the relief valve in the hydrostatic front pump they look fine the screen has been cleanend out bouth drive motors have been gone though so I don’t know where else to look