A buoyancy compensating device is disclosed. The buoyancy compensating device includes an air chamber for containing gas for controlling buoyancy, a vent valve for releasing the gas from the air chamber, a tube secured to a shoulder strap of the buoyancy compensating device, and a pull knob for operating the vent valve. The pull knob includes a head piece and an elongated tail piece extending from the head piece. The tail piece is extended into the tube and connected to the vent valve. The tail piece is configured to reside in the tube while the pull knob is pulled by the diver for opening the vent valve. The tail piece may be connected to a pull cord which is then connected to the vent valve. Alternatively, the tail piece may be directly connected to the vent valve.