Trolley Jacks.

Anyone know anything about hydraulic trolley jacks? I have two; a little lightweight one and a decent two-tonne job. The latter seems to be reluctant to lift to full height - the pressure stroke at higher lifts has become very short with little result. I'm assuming that it needs the fluid topping up at least, but I really don't know how to set about it. It isn't obviously leaking.

Chris.