A method of controlling fluid pressure in a closed system including using a pressure regulating unit connected to the system, the unit including a first conduit fluidly connecting the closed system to an external system. A first regulating valve is interposed between the external system and one end of the conduit and a second regulating valve interposed between the closed system and the other end of the conduit. The method further includes: a) opening one of the first or second regulating valves; b) closing said one regulating valve; c) opening the other regulating valve; d) closing the other regulating valve; and e) repeating steps a) to d) in turn until the desired fluid pressure is achieved in the system.