A system (32) includes a first valve (10a) fluidly connected to a first vessel (30a) and a second valve (10b) fluidly connected to a second vessel (30b). The first valve (10a) includes a body (12) and a piston (18). The body (12) includes first (20) and second (22) ports and a bore (14) having a longitudinal axis (16). The first port (20) is in communication with the bore (14) and an interior of the first vessel (30a). The second port (22) is in communication with the bore (14), the second valve (10b), and an atmosphere (52) exterior to the first vessel (30a). The piston (18) is movable along the longitudinal axis (16) of the bore (14). A first position (FIG. 1) of the piston (18) blocks the first port (20); a second position (FIG. 2) of the piston (18) allows fluid communication (36) between the first (20) and second (22) ports. The first valve (10a) is configured so that fluid pressure (34) from the second valve (10b), communicating through the second port (22), urges the piston (18) to the second position (FIG. 2).