apologies for the delay - for some reason I am not getting reply-alerts.
the existing sensor is part of a bulky device that also contains the physical cut-off that I am trying to avoid. I've had no success in disabling the cut-off.
by using the term 'mimic' i had intended both to ask for help with a low pressure sensor generally - is there something available in the hobby price range? and also to invite lateral ideas about how to achieve the same goal (i.e. that the water would turn on when a tap was opened, and shut off when closed).
If anyone has anything that might help on this I'd be much obliged.