A cleaning system of a combine harvester includes a fan, a first valve member positioned within a first duct of the fan for throttling the flow of air through the first duct, a second valve member positioned within the second duct of the fan for throttling the flow of air through the second duct, a first actuator for adjusting a position of the first valve member, a second actuator for adjusting a position of the second valve member, first and second sensors for sensing air pressure at first and second sieves, and a controller. The controller is configured to receive the sensed air pressures from the sensors, compare the sensed air pressures with either each other or threshold values, and transmit instructions to one or both of the actuators to adjust the positions of one or both of the valve members.