Abstract: A pressure regulatory valve system for a breathing system in a deep diving apparatus comprises a pair of pressure relief valves, one adapted for connection to a gas supply line extending from a source of breathable gas provided by a diving bell to the diving helmet of a diver working out of the bell, and the other being adapted for connection to a gas return line extending from the diving helmet to the bell. The relief valves are adapted to be controlled by differences between the pressure in the helmet and the pressure in the bell so that when the diver moves to levels above that of the bell the relief valve connected to the supply line responds to reducing helmet pressure in an opening manner and vents gas from the supply line to the bell, while the relief valve connected to the return line responds in a closing manner.