A ball valve has a valve body having a chamber, aligned inlet and outlet passages providing flow therethrough, and an access passage communicating with the chamber. A replaceable cartridge assembly is seated in the chamber and includes a generally spherical ball having a flow passage therethrough alignable with the inlet and outlet passages, an inlet valve seat, and an outlet valve seat. A cartridge body member extends about and couples the ball and the valve seats. A resiliently deflectable biasing member biases the inlet valve seat and the ball against the outlet seat. A resiliently deflectable biasing member acts upon the inlet end of the cartridge body and a deflector is movable between a first position in which the biasing member is substantially free from deflection and a second position in which the biasing member is deflected to bias the cartridge body and the outlet valve seat against the wall of the valve body chamber to effect a seat-to-body seal.