I'm almost positive that your lift is 100% Nissan. If the lever which works the tilt does indeed move the spool in the hyd control valve & the tilt cyl's don't move I would highly suspect the control valve as being internally faulty. Lifts as old as yours don't usually have any electric control spools on the control valve- you should check to confirm this.
If there's no electric spools & the tilt spool is free & sliding in it's bore than a rebuilt control valve would be my choice. I wouldn't disassemble the control valve & try to repair- there are no parts available & the liability would be over the top
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