1995 Windstar intermittent Brake Binding

I have a 1995 Windstar that I am currently having brake trouble with that is hard for me to pinpoint the failure. The vehicle has 115,000 miles and is on its third transmission, had head gaskets replaced, but
is currently running Ok with the exception of the brake system. The front pads are in decent shape and have about half their life left, rear shoes about the same.
The problem is that intermittently the brake pedal binds on the way down. You can let the pedal up and pump it once or twice and the bind goes away and the pedal is back to normal. I also here a little twang like a spring type noise once in a while when it fails. It will then work for at least the next 25 brake actions and then fail again the same way. I originally suspected some type of bind in the calipers or somewhere in the rear shoe mechanisms but I took the pads, calipers, shoes, and springs all off inspected them and put it all back together. After this it fooled me for a while and the failure didn't occur for a while but eventually it came back. I think my actions did not really affect the problem. I tried to make it fail by starting the car and pumping the pedal with my hand but I could not get a failure this way. I just took it into the local garage and they claimed they couldn't get it to fail. When I described the failure to them they did not suspect any failure at one of the wheels but rather a mechanical pedal failure or a master cylinder failure. They could not isolate anything and said the pedal mechanism looks fine. They said I could take a shot at this being a master cylinder failure and have them replace it but they would not guarantee it was the fix. I held off on this when I was thinking of the expense at a 'wishful' fix. My experience with doing this has been you encounter new issues with an old car and after many more $$$'s then anticipated the original problem still exists.
I am hoping someone has had a similar problem and can offer some advice.
A second question I have is does this sound symptematic of a master cylinder failure ? I was under the impression that if the master cylinder was bad it would be a more solid failure and I would get no pressure. In this case the car stops fine after a couple of pumps at the pedal following the bind.
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Frank Gassert wrote:

does not look any different, just its actions are different....
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