Abstract: Sudden increases in fluid pressure which could be injurious to the diaphragm and seals of a sensitive capacitive pressure transducer, or the like, are instantly protectively blocked or "checked" automatically by a miniature naturally-powered valve having a low-mass one-piece valve member or plug molded of elastomeric material which is shaped and disposed to be immediately propelled into guided tight line-contact seating and sealing as an abrupt pressure surge commences and then to hold such sealing only until the plug is unseated by gravity when either the pressure is rapidly lowered suitably in operation of the transducer or the diaphragm is slowly exposed to increased pressure after enough minute normal valve leakage takes place.