If there is no loss during your pressure tests I would put capacity in car, run it up, mark where sill is, pull and lock-out disconnect, check location to see how fast if any it's sinking, if no sign let set overnight. From there just eliminate where your loosing it. Shutting the gate valve off while having a gauge in will show if it's the valve. Of course if it still sinks with gate valve shut it's pipes or cylinder. Be aware, I've also seen the jacks fill.