Abstract: A large vacuum pump is connected through a vacuum line to the cow milking apparatus, and a vacuum regulator is employed to admit into the line the amount of ambient air necessary to maintain the line pressure (and thus the pressure at the milking apparatus) constant within very narrow limits. The vacuum regulator comprises an air valve to control the admission of ambient air, and which is itself controlled by a light bias spring (having a low spring rate) and an opposed diaphragm. One side of the diaphragm is subjected to pressure generally equal to line pressure, whereas the other side of the diaphragm is subjected to pressure of air within a stabilizing and regulating dome. Adjustable means responsive to the pressure differential between the dome and ambient are provided to admit into the dome sufficient air to maintain the dome pressure constant. Furthermore, means are provided to bleed air through the diaphragm, thus maintaining substantially constant the pressure drop thereacross.