Brakes-Master cylinder humming?

[weasel]

Sorry nothing to do with the power steering (as posted before). The humming/whirring sound is definitely coming from the master cylinder unit. Any ideas what could possibly be going on in there?

W

[Llanigraham]

There isn't anything in a master cylinder to make a hum. I would suggest that the noise/vibration is elsewhere and just appears to be coming from there.