valve body member 28 the bottom edge of this front face 40 forms a straight flexure edge 42 against and along which the tongue section 32 of valve body member 28 flexes under the force of air flowing upward through vent hole 16 this bottom straight flexure edge 42 of front face 40 of plug 36 likewise extends transversely across recess 20 at an angle which is not perpendicular to the centerline cl of the valve body member 28 on the other hand the vent hole 16 is centered between the opposing walls of recess 20 so that the center c of hole 16 lies on the centerline cl as best seen in fig7 because of this nonperpendicular orientation of the flexure edge 42 the vent hole center c is offset from a line pl perpendicular to the flexure edge 42 at the point p at intersection with the centerline cl this line pl represents the flexure direction of the flexure edge 42 of plug 36 and the vent hole 16 may thus be considered to be eccentrically located relative to this flexure edge 42 the eccentric orientation of the vent hole 16 relative to the flexure edge 42 of plug 36 is to be contrasted with the prior art orientation shown in fig4 6 wherein a vent hole 16a is centrally located relative to the flexure edge 42a in the prior art arrangement the front face 40a of a plug 36a together with its bottom flexure edge 42a extends transversely perpendicular to the centerline cla of the valve body member 28a so that the center ca of vent hole 16a falls along a line pla perpendicular to the flexure edge 42a and coinciding with centerline cla the operation of this prior art valve is illustrated in fig4 6 in.