A vertically reciprocable gate assembly which includes a frame surrounding a port and a gate having a gate member connected by links to an actuating member which is slidably connected to the frame with the gate member between it and the frame. Unless the gate is closed, the gate member is held against the actuating member and out of sealing contact with the frame. When the gate is lowered, it engages a stop which establishes a predetermined position of the gate member relative to the port, further downward movement of the actuating member causes the links to thrust the gate member into its operative position.