I would suggest checking the valves on top of the pump unit. They should be backed off, or turned counter-clockwise.
Second, you should check the fluid level, but only when the jacks are retracted. They take MD3, the GM automatic transmission fluid. You could change the fluid. Just siphon out to old and pour in the new. It takes about a gallon, if I remember correctly. I did it just as a preventive measure.
Third, wipe the exposed piston on each jack with MD3 fluid, or spray with silicone spray. I do this every time I extend the jacks just for preventive reasons. My HWH system is 20 years old now and still works great.
Holiday Rambler Navigator DP, Hummer, and Honda VT1100C Shadow