Any developments Dave?
Offline,
You may not need a demand valve unless you intend to carbonate your keg normally
and use the keg pressure to push the beer into the Engines.
If you have not already done so have a read of the whole "Show us your Beer Engines " thread.
If you have already done so I will shut up.
i have 2 Beer Engines and use minimal keg pressure and an aspirator to control the quantity and
pressure of CO2 into the keg when the Engine is in use.
Regards
Graeme