Can't say for sure what type pump they used on the hydraulics on that machine, but I know the vane pumps CAT often used on their equipment would lose their prime if left setting for a period of time. Too, I have had the vanes stick and not want to pump properly right off.
The first thing I'd do is look on the inlet or the outlet side of the pump and see if there is a small fitting/plug that you can remove. With the plug out pour in a quart or two of hydraulic oil, and replug the hole. I've had to repeat this proceedure several times on occasion to ever get the pump to actually pick up, but give a couple of shots of oil, it never failed to work.