Pressure reducing valves (PRVs) are widely used in water distribution networks for optimal pressure management. Several studies and field applications prove the effectiveness of PRVs for water loss reduction, but some recent studies have also highlighted some problems and operational limitations. In this study, we analyse the functioning of a piston actuated pressure reducing valve (where the regulating device operates in a parallel direction to the flow) with a mechanical pilot which is subjected to low flow regimes, a condition that is often observed in the real distribution networks. The analyses are carried out by means of laboratory tests featuring two sets of experiments, i.e. a) by testing the behaviour of the PRV when a pre-established initial value and subsequent variation of flow rate occurs in the system and b) by testing the PRV in face of a temporal series of flow rates observed at the inlet section of a real hydraulic district. The first set of tests highlighted different field behaviour of the PRV corresponding to well defined ranges of flow rate, and in particular that for some flow rates the PRV tends not to respect the imposed set-point value and, under certain flow rate values, an unstable behaviour characterised by significant pressure oscillations occurs. The second set of laboratory tests has shown that the anomalous behaviour identified in the first set of tests can occur in ordinary operational conditions of a network, implying potential technical and economic consequences in terms of damage to pipes and hydraulic devices.