I have a late 90s Ransomes Frontline 728D 4WD hydraulic riding mower that won't move under its own power. These are its current symptoms: The deck still raises and lowers, and the blades spin. Pressing the pedal in either direction causes the mower to move about an inch, then it stops. I've lifted the front wheels off the ground one at a time, and they both spin forward and backward with decent speed when the pedal is pressed in both directions. I've also lifted both rear wheels off the ground together, and when I press the pedal forward, they turn forward just a little and then stop. They do not move at all in reverse. The manuals that I've found say that these mowers are 2WD in reverse, so the rear wheels not moving in reverse is normal.
This mower was purchased a while ago as a non-functioning unit. Parts that I have replaced so far include the anti-reverse check valves for the rear wheels; the right front drive motor, which had been locked up because a thrust bearing disintegrated and the rollers jammed the rotor; and the pistons inside the main pump, which had stuck shoes. And I've changed the oil and filter, of course.
I think I'm close, but I've run out of ideas of what to do next. This is my first hydraulic mower, so I don't have any real experience with them and there very well could be some simple little thing I've missed. Any advice would be greatly appreciated.