A grain unloading system for a combine harvester comprises an unloading tube having a discharge outlet and a helical auger situated in the unloading tube and operable to convey grain towards and through the discharge outlet. A dribble control door serves to inhibit the leakage of grain from the tube when the auger is in a non-operative mode. The door is pivotally connected to the tube adjacent the discharge outlet so as to be pivotally moveable between a closed position and an open position. A lever arm is fixed at one end to the dribble control door, and at the other end to a gas spring which is mounted to the tube. The gas spring is pressurised so as to provide a closing moment to bias the dribble control door into the closed position. The geometry of the lever and gas spring is such that the closing moment increases as the dribble control door moves from the open position to the closed position.
