An IOP control system (200) provides drainage from an anterior chamber (340) of the eye to a drainage location at the eye. The system has a valve (230) with an open flow position and a closed zero flow position. A sensor system (210) includes a first sensor (PI) arranged to detect a first pressure representative of IOP. A controller (200) receives data representing the detected pressure from the sensor system and compares the data to an upper pressure threshold and a lower pressure threshold to determine whether to change the state of the valve system in a bimodal fashion. A second pressure sensor (P3) is located remotely in a manner to measure atmospheric pressure.