I am trying to diagnose a strange problem with the brakes on my 2001
Ford Windstar. When the pedal is depressed, we hear a sound from under
the dash board that kind of sounds like something rubber being twisted
or stretched, kind of a loud rubbery sqeaking sound. This isn't the
pads or shoes. Definitely inside the car. I have the Ford Extended
warranty and the dealer has replaced the brake booster three times so
far. However, that hasn't made any difference at all. They tried to
tell me that it's normal but it didn't do it when we bought the car and
only started in the last 18 months or so.

Supposedly, it's the rubber bellows inside the booster making the
noise. But, since we're on our third replacement so far, I'm beginning
to wonder. What else could be causing this noise? If it is coming from
the booster, is there anything else that could be causing the booster
to make that noise?  I found a post on Edmunds that suggested that the
problem is actually with the master cylinder, but can't say for sure.
And, I'm not sure how to convince the dealer that that is what the
problem is.

It's worse when the weather is warm and when the problem really gets
bad, the brake pedal gets hard to press, as if there is a lot of
resistance.