Windstar brakes screeching

have a Windstar 2000, mileage around 35,000.
if i drive in rain, the next day when backing out of my garage the brakes screech tremendously when i depress them. i'm not talking about a little
squealing, this is ear shattering screeching.
had it to the dealer when it was under warranty and they said their was moisture present. they did something, although i can't remember how much it fixed the problem.
it only happens after it rains. as soon as i get to the end of my driveway (hoping that i don't wake up the neighbors) and start going forward the brakes are completely silent. also, once i get to work if i back up the brakes are noiseless.
questions:
1) any idea what the cause is 2) why does it only happen after it rains 3) why does it only happen when backing up initially
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