Interesting thread...

Andy, air getting in seems to be a common problem. Is there a way to allow any air that gets in to escape or vent to a trap? I don't know much about hydraulic systems but I would guess without a circulating pump and system that it may not work unless purged each time etc, so is there a way to let it simply ascend and be trapped above a valve that would vent but close once pressure is applied through the system that is "below" the vent valve? I can make a sketch if this is too confusing, I hope the idea makes sense even if it may not solve the problem.

Good luck,
Dave