Abstract: The fluid forced by a pump 1 through a conduit 2 towards a point of use such as a spraying bar 6, passes through a gate valve 7 which generates a pressure drop. The higher pressure P0 and the lower pressure P1 thus determined act on a diaphragm 10 which carries a flap valve member cooperating with a seat 18 formed at the inner end of a discharge tube 15-5 which returns the liquid to the tank from which it is pumped. An auxiliary diaphragm 17 acts in the reverse direction. As soon as the pressure difference P0 - P1 exceeds a given value, the flap valve member 14 closes. It is easily demonstrated that under such conditions the delivery pressure P1 is proportional to the pressure drop P1-P0, irrespective of the flow rate through the nozzles of the sprayer bar 6. If 1 is driven at a constant speed, P1 therefore remains constant.The invention is particularly applicable to agricultural sprayers.