A method for the production of a valve is provided comprising molding a valve body with a first mold, preloading the valve body with a second mold, molding a sealing portion in an open position onto the valve body, and releasing the preloaded valve body. The sealing portion enters a closed position when the preloaded valve body is released. When the valve is in use, the sealing portion opens when under a fluid pressure, and closes when the pressure is released.