A method and system is intended for fine proportioning of fluids injected into a pumping installation, e.g., a liquid chromatography installation, allowing to compensate for non-return valve errors. The system includes a fluid delivery system including vessels for constituents (A, B, X) that communicate with a pumping unit (1-11) by means of a one-way valve (8) that opens upon suction, and a control unit (PC) that adjusts the respective proportions of the various fluids sucked by acting on the opening time of solenoid valves (EV1 to Even) associated with the various vessels. As the opening and closing times of the one-way valve are most often unequal in practice, control unit (PC) is suited to cyclically change the order of the control signals applied to the various solenoid valves during the various successive suction stages of each pumping unit so as to compensate on average for the proportioning inequalities due to each one-way valve.