A purging device for an assembled unit for a vehicle, having both a wheel comprising a rim and a tire. The purging device comprises a body connected to the inflation tube of the assembled unit, said body being designed to be subjected to the inflation pressure of the assembled unit, and a piston able to move inside the body, said piston operating an open/close element, fixed to the piston, between a closed position and a purging position of the body, said open/close element being normally closed, the open/close element and the piston each having a face designed to be exposed to the inflation air inside the body in the closed position, and the open/close element being opened by the action of an operating force on the piston. Also disclosed is a method for deflating an assembled unit.