A fluid-flow control valve includes an inlet (E) for connecting to a feed duct; an outlet (S) for connecting to an evacuation duct; a valve member (1) that, at rest, bears against a valve seat (2) to isolate the inlet (E) from the outlet (S); and an actuator mechanism (5, C) for moving the valve member (1). The actuator mechanism includes a chamber (C), which communicates with the inlet (E) via a hole (11) and is disposed relative to the valve member (1) on its side facing away from the seat (2), in such a manner as to urge the valve member against its seat; and a bypass (5) that connects the chamber (C) to the outlet (S), the bypass being controllable between an open state for passing fluid, and a closed state.