Drop in IP is entirely normal and it is the drop in IP that provides the force needed to lift the seat off the orifice in the first stage.Tim Wong:Another question here. Would there be an effect on the IP pressure (and by how much) when air is drawn from the second stage? Thanks.
How much drop occurs depends on the flow capacity of the first stage and on how much air is being drawn through the second stage. A normal inhalation produces relatively little drop, while a full depression of the purge button will normally produce a larger drop.
What is more important is that the IP locks up tightly after the inhalation stops and no IP creep occurs.