Abstract: A control device designed to be responsive to a change in the rate of flow of liquid in a flow line, for example milk in the flow line of an animal milking machine, between a relatively high value and a relatively low value, including a vessel through which the liquid is arranged to pass and in which it can attain different levels, and a control member movable in response to the change in volume of liquid in the vessel between a first position into which it is biased when the vessel is empty, and a second position into which it is arranged to move when the vessel is full, the vessel having in its lower part an aperture through which liquid can flow out of the vessel, and the arrangement, including the size of the aperture, being such that when the liquid flow rate changes from the relatively low to the relatively high value the vessel fills and the control member, which may be the vessel itself or a float, moves to its second position, and when the liquid flow rate changes from the relatively high to the rela