A connection between a vacuum source and a conduit of a milking machine is controlled by a valve body having a position for providing a normal working vacuum in the conduit and a position for providing a reduced working vacuum in the conduit. The valve body is actuated by an element movable in a chamber in response to a pressure change in one part of the chamber communicating with the vacuum source through a passage; and a regulating device is provided for opening and closing this passage to cause movement of the valve body from one position to the other by the actuating element. The regulating device may be under the control of a milk flow indicator, so that the reduced vacuum is provided automatically whenever the milk flow is below a certain rate.