I think you will find the seal (and most likely the pump itself) to the main (lift) hydraulic pump is gone bad, on this truck (if memory serves correctly and I am getting old, but so is your forklift) the lift pump is driven off a a gear in the transmission housing. This is usually the only way you manage to 'make more oil'.
There is a 'pickup' with a filter screen, on the line at the bottom of one side of the transmission housing, that may be worth checking, you will need to remove the fitting into the transmission to find the screen, as it extends about 3 or 4 inches into the transmission..
If your truck has the Peugeot engine (that has the spark plug wires going down in the middle of the valve cover, there were more than 1 engine option for these trucks), your truck is not worth (could not be sold to a wholesaler) for what the pump costs, and may be time to consider it scrap.