The pressure regulator has a regulating valve (V) in a vent opening (17) integral with the airbag (14). The regulating valve has a valve gate which can be selectively opened and closed. A spring (26) biases the valve gate to the closed position. If the gas pressure of the airbag exceeds a set value, the vent opening is opened. When the gas pressure is reduced to a set level, the valve closes to retain the bag half inflated.
