I had that same problemon my 2001 passat avf. You have to take that plastic cover,that covers brake servo,battery etc. There are water draining holes and after while,there are lots of rubbish,like leaves,deust and so on and thats blocking drains. I have cleared drain there,cleaned out all rubbish,dried all water with rags and problem went away. I maybe wrong,but water probably was finding way inside servo somehow and like you mentioned before,brake pedal was going really stiff and to stop the car i had to press it really hard. It happened to me on my way home after buying that passat. I was approaching round about and as usually i was kinda pressing brakes couple times,just to slow down a bit,but when i was near round about and needed to slow down to almost stop,that pedal gone really hard and brakes were not good,well they were ok,just pedal had to be pressed really hard. That was a bit scary,because i wasn't expecting that to happen. When got home,i spoke to my mate,he had same car and he said that his car was doing same and he offered me to do what i described in the beginning of the post. That pedal didnt give me no more problems til i sold car and done 40000 miles in 3 years.
It won't cost you anything,but might help.
Good luck and hope you will get it sorted.
Brakes, vacuum and servo pipes
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