An air distribution apparatus has a manifold body, and ports defined through walls thereof, with a delivery conduit connected to each port. A port valve is configured to connect and disconnect the delivery conduit to the interior of the manifold body. A supply conduit delivers a product air stream with agricultural products entrained therein to the interior of the manifold body. The supply conduit extends vertically up from the manifold to a curved elbow and then extends horizontally from the elbow. An exhaust orifice is defined in the inner radius of the elbow. When an exhaust valve is open, a selected flow of pressurized air flows from the supply conduit through the exhaust orifice. The port valve and exhaust valve are controlled such that when the port valve closes, the exhaust valve opens. The exhaust air can be directed into the delivery conduits to clear product therefrom.