Abstract: A preferred embodiment of FIGS. 1-3 includes a check valve (10) with a pair of semicircular check valve members (34, 36) normally permitting fluid flow in one direction and blocking fluid flow in an opposite direction. Semicircular valve member (36) is releasably locked in an open position to permit a back flow upon manual rotation of handle (46) after manual gripping of knurled knob (84) and withdrawal of a locking pin (82) from a receiving opening (96). A safety lever (98) is pivoted out of a covering relation to knob (84) before knob (84) is gripped and prevents rotation of shaft (38) unless lever (98) is manually pivoted out of covering relation with knob (84). Another embodiment shown in FIG. 4 permits selective opening of a semicircular plate valve member (34A or 36A) to a releasably locked open position. Manual rotation of handle (46A) in one direction effects movement of semicircular valve member (36A) to an open position.