A unidirectional fluid valve for use with a respirator includes a flexible flap initially formed as a flat resilient member. A valve body has a flat valve seat to support the flexible flap. The flexible flap is contoured to make contact with the valve seat when a fluid is not passing through the valve body, and free to lift from the valve seat when a fluid passes through the valve body. The valve body additionally includes an arm located off center relative to the flap and the arm extends inwardly in the valve body to contact the flexible flap off center to push the flexible flap at a position to deform the flexible flap to contour a concave configuration for the flexible flap and provide a secured end and a free end for the flap. In this way the flexible flap is normally pushed into sealing engagement with the flat valve seat when fluid is not passing through the valve body and is free to move away from the valve seat when fluid passes through the valve body.