Backflow check valve

A backflow valve comprising a valve seat, a valve flap, and a flexible hinge, and adapted to be installed and maintained in an unmodified conventional cleanout wye through the side channel thereof without removing said wye from service, wherein the valve seat has an exterior tubular wall with upstream and downstream portions sized for insertion through the cleanout channel, said upstream portion has a diameter slightly less than that of the inside tubular edge of the upstream edge of said wye, and said upstream portion has attachment and sealing means to secure said valve seat to said inside tubular face of the upstream channel of the cleanout wye by inserting the valve seat through the cleanout channel of said wye and forcing the valve seat upstream a short distance, thus engaging said sealing means between the upstream edge of the exterior tubular face of the valve seat and the inside tubular face of the upstream channel of the cleanout wye, said attachment means consisting of at least one tooth which protrudes from the upstream portion of only the exterior tubular wall of the valve seat and which lodges against the interior tubular face of the upstream channel of the cleanout wye when the valve seat is installed therein to prevent movement of the valve seat with respect to the cleanout wye after said valve seat is installed, said sealing means consisting of an O-ring which seats between the upstream portion of the exterior tubular wall of the valve seat and the inside tubular wall of the upstream channel of said cleanout wye, whereby said valve may be installed without adhesives or other external securing means, said hinge being backed by a shoulder curved on a large radius.