A valve body and seal assembly comprises a valve body having one or more circumferential seal retention grooves and a dual-durometer elastomeric seal cast-in-place in at least one groove. Each seal comprises first and second sections in contact along a substantially non-planar intra-seal interface. First seal sections typically comprise polyurethane having a higher modulus than the polyurethane typically comprising second seal sections. Residual elastomer shrinkage stress present after curing of a cast-in-place dual-durometer elastomeric seal is redistributed within the seal sections to maintain beneficial preload. Seal configurations described herein are also associated with decreased shear-related elastomer fatigue and extrusion damage associated with repeated valve seat impact. Valve service life is increased by beneficial preload maintained by the synergistic interaction of first and second seal sections.