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Also,clamp off the pipe between the pump and servo and see if this stops the hissing and see if the pedal still creeps down.If it stops the noise then I suspect you have servo problems!(Don't drive it with the pipe clamped though!).
There is also a one way valve into the servo from the pipe.If it's easy to access,remove it and see if it's faulty(It will allow air to be sucked out of it but not blown through it.Ooh Er!!)
I'd rather push my Alfa than drive a BMW....
Currently in Lagdti's garage:
2004 Saab 9-5 HOT Aero and a 2001 Audi A3 1.8 Turbo