Blown cylinders are common on older hydro units. If caught early it usually will not fail catastrophically, but it can so don't put anything you want to keep, like your body parts, in tight spots. It usually involves a small leak and doing a static/ relief test while monitoring the oil is the easiest. As Montana described is the static test, Confirm the oil is not leaking out of the head. putting oil absorbent pads around cylinder below the head is an easy method. Confirm the piping is not buried as it may be what's leaking. Keep the shut offs closed during the static test. If the unit was buffered for an extended time the packing may relax and siphon the tank dry, even with the shut offs closed. Guess how I know that. If oil is missing something leaked and if you cant see what leaked then it probably leaked under ground.
