Abstract: A pressure relief valve such as is used to relieve over-pressure in the passenger compartment of an automotive vehicle has valve flaps which are retained in connection with a housing by molded-in-place retainer strips. The valve is manufactured by an injection molding tool having a pair of mold halves defining a first mold cavity for forming the housing and a second mold cavity for forming the flaps. After the housing and flaps have set, the mold is opened, with the housing remaining in contact with one mold half and the flaps remaining in contact with the other mold half. One mold half is then moved relative to the other to place the housing and the flaps in alignment with one another, and the mold halves are closed again to place the flaps inside the housing and define a third mold cavity extending along the lines of desired attachment between the flaps and the valve seats.