Abstract: A vault door lock of a combination type that operates on fluid pressure. The locking system has a master control valve assembly mounted on the rear face of the vault door and operatively associated with variously positionable dials mounted on the front face of the door and reciprocally mounted actuators which pass therethrough. A slave valve mounted on the rear face of the vault door is operated on command from the master control valve. A plurality of locking members are movably mounted on the vault door from one to another of a locking and unlocking position and coupled to the pistons of a piston cylinder assembly for moving the same. The master control valve consists of a plurality of operatively interrelated valves designed to provide an output that actuates the slave valve when such interrelated valves are actuated in a predetermined sequence. An alarm trigger is actuated by the master control valve when the operatively interrelated valves are actuated in a sequence other than the predetermined sequence.