The pump has a restrictor valve (42) located in the supply channel (36) and is mechanically operated by a float (46). The restrictor is a flap valve which can pivot around an axis (A) which extends perpendicularly to the centre axis of the supply channel through the channel's middle. In the closed position the flap valve is inclined to the axis of the supply channel. The float is connected to the flap valve by a lever (44) which can pivot around an axis that is perpendicular to the axis of the supply channel.
