A gate valve assembly having a body, a gate, a stem, and a backseat for sealing between the stem and the body. The backseat is an annular member with a complementary surface on its inside that fits over a spherically shaped section of the stem; thus allowing the backseat to rotate and pivot with respect to the stem. An outer surface of the backseat is beveled for sealingly engaging a stem passage formed through the body. A flex member is provided that is for sealing a surface adjacent the backseat. The flex member can form a seal between the backseat member and the stem or between an annular retainer and the backseat.