i have had this on my red golf it has got stedily worse despite two system bleeds its still taking on air somewhere .... so i am driving my white one until my man theiman can find the fault.
Jet 
Jet, maybe your servo is buggered?
The inner diaphram split on mine so was taking in air when i braked. Gradually got worse till it sounded like lorry air brakes.
Eventually i lost all brakes, car would stall as soon as i touched pedal.
Nice one SI I will have it looked at
Jet
