A mud saver valve for retaining drilling fluid in the Kelly of a rotary drilling rig for insertion into a Kelly sub with an enlarged opening inside having a valve and hollow piston closing the flow in a first position and having flow through when the hollow piston is moved by flow against a spring to an open position compressing a spring to stack height, the spring being magnetized to provide magnetic latching to assist in holding the hollow piston in the open position at flow rates lower than it would otherwise be held open.