The present valve design consists of two leaflets that are freely movable around a pivot and arranged within an orifice. At the open position the leaflets form a slight divergence angle. The flow patterns at low Reynolds number generated by such a design are associated with leading-edge stall and vortex-shedding from the leaflets. The present investigation is directed to a better understanding of the nature of this flow. In particular the effect of leading-edge stall on the angular oscillation behaviour of a single leaflet is of interest. For this purpose the leaflet momentum has been described in terms of a simple nonlinear hydrodynamic model which takes into account the direction of the gravitational vector. Based on that model the frequency behaviour was investigated numerically in parameter space. The effect of the flow speed on the oscillatory leaflet motion is treated as quasi-steady, that means for small Strouhal numbers Sh ≪ 1. The preliminary results reveal a strong dependence of the oscillation frequency on the amplitude and on the direction of the gravitational field vector. On the basis of a perturbation analysis an expression relating frequency and amplitude was found.