Very worrying. Start engine then depress brake pedal to engage gear box Pedal slowly depresses to the floor squealing all the time.(( Squeal is definitely coming from under bonnet in the area of the master cylinder )). When fully depressed then allow pedal up and depress again squeal stops and pedal slowly returns to top of stroke after a couple of depressions. Very similar to having air in the system only pedal does not depress when under pressure to depress once top of stroke is achieved..Also only happens after car has been left unused for several hours..