Appears from as many of these old Grove units out there the manuals should not be hard to find, well that's wrong. Sure could use the manuals for the unit. The unit has about 5400 hours and it's been used a bit. Don’t know if it’s an A, B or C model but the serial number is 806021. I saw a pic of a B model with the counter weight hanging under the back, mine has the weight built into the back, control panle on the right side. The things I've found which need fixing is the platform hydraulics will turn left but not right. I think the solenoid is behind the stick control panel and by manually pushing the solenoid I can make it more left but not right. The high speed switch does appear to work but that’s OK, don’t plan to go that far, but would like to figure where the control is. Took the oil filter off and I have never seen one with just two little holes 180 degrees across from the screw on pipe, what kind of filters is it? Most everything else appears to work.
swap the hoses and/or the wires leading to the valves/solenoids and see if the problem follows the hosing/wiring or stays with the direction. Do you have photos?