(1) It might need to be bled. There is a bleed nipple for clearing air out of the cylinder. Oil is incompressible for all practical purposes whereas air is very compressible. Hydraulics depend on the incompressibility of the fluid.
(2) The cylinder could be leaking - no hydraulic fluid.
(3) Is the check-valve fully closed/open?
Hydraulics work on the principle that pressure is transmitted uniformly in all directions in a fluid. If a small force is applied to a small diameter (and therefore small area) piston it creates a pressure (which is force divided by the area over which it acts). If this small piston is connected directly to a much larger cylinder and piston, the same pressure is acting on the larger piston as is acting on the small one - because it's the same fluid. However this pressure is applied over a bigger area and since force equals pressure multiplied by area, a much bigger force results. In short you apply a small force to a small cylinder whne pumping the trolley jack handle and this results in say 2 tons of force in the lift cylinder.
(You did ask - so don't complain about the theoretical lecture!!)
JD, February 2008
