As David points out wih factory info letting the pressure off via your regulator, drop the car to low running and put a 12mm spanner on the bleed screw, with the nut, turn counter clockwise half a turn, you'll hear it hiss, hopefully you'll know this already. You can shake a bit more out by rotating the steering once left or right and back full then to straight. I mention that because I remember some serious citroenistes from NZ doing a lap of Oz who had to fix a leaking pipe and they put car on club hoist and went at it and got a good dose of eau de vert minerale mist all over them. Then they looked a tad surprised as it dawned on them what they had forgotten to do. Just saying ...