A precleaner for use in an air intake system is described, the precleaner comprises a cyclone which defines a volume, the cyclone comprising an inlet and an outlet. The inlet and the outlet are arranged to generate a vortex within the volume of the cyclone when the air pressure at the outlet is lower than the air pressure at the inlet. The cyclone further comprises an aperture. The precleaner further comprises a blocking member arranged to move and concomitantly occlude the aperture only when the air pressure at the outlet is lower than the air pressure at the inlet. An air intake system comprising the precleaner is also disclosed.