A flow modulator system for a pressure reducing valve includes means to sense changes in flow and actuate the valve in response thereto. A first means (3) to create a first differential pressure is arranged in the flow path downstream of the valve, said first differential pressure being dependent on the flow rate of fluid in the flow path. A bypass tube (6) of smaller cross-sectional area is connected between a location substantially adjacent to a low pressure zone (4) of said first differential pressure creation means (3) and a point (5) between the valve (2) and the first differential pressure creation means (3). The bypass tube (6) is provided with a second means (7) to create a second differential pressure, the pressure at a low pressure zone (8) of said second differential pressure creation means (9) being transmitted to the valve (2) as an indication of the flow rate in the flow path.