A valve with a stem that may be mechanically actuated to two different positions by rotary or linear motion. in the first position, one side of the valve disk is located against the seat permitting movement of the disk in a manner that will allow a flow of fluid in one direction through the valve. in the second position, the valve disk is lifted from the seat to allow a flow of fluid in either direction through the valve. the valve disk may be spring loaded. see also valve, stop-check and valve, stop-lift-check.