Abstract: A device for automatically disabling operation of a pneumatic device, such as an air-actuated pump switch. The device includes a diaphragm exposed to atmospheric pressure on one side and to water pressure on the other. So long as the water pressure is high enough, the diaphragm is held in a position in which it seals a pneumatic port and ensures continuing operation of an air-actuated switch. If the water pressure falls below a preselected level, the port is vented to atmosphere and the air-actuated switch is effectively disabled until the water pressure increases again.