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.
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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