A valve assembly for a plug to close off a zone in the well for further treatment above the plug features a valve member initially pinned in an open position where flow up to a predetermined level can move through without dislodging the valve plug to move to its associated seat. If the perforating gun misfires a replacement gun can be run in with flow as the plug is still retained in the position for flow through the plug. On the other hand flow around the plug above a predetermined level will shear retainers for the plug and let the plug land on its seat closing flow in a downhole direction for treatment.