Brake problem with a 2001 Ford Winstar?

I have a 2001 Ford Winstar that keeps having "squealing" brakes after it rains. The noise could be described as more of a high-pitched ringing than a squeaking sound. There's no sound of grinding. The front rotors, pads, and calipers were all replaced in April, this problem started occurring about 2 weeks ago. I only hear a squeaking noise when I'm pressing gently on the brakes at a slow speed, such as pulling into a parking spot. If I press harder the noise stops. When the noise is occurring, the car simply does not stop as well. When that happens, the brake light comes on and the ABS system kicks in. I checked my brake fluid, it was just barely above the "minimum" line so I topped it off with manual-recommended DOT 3, did not seem to have an effect. Whenever it rains, the problem is consistent with the car for 24-36 hours, then the brakes are fully functional until it rains again. I'm thinking maybe I need a brake flush but I'm having a hard time finding any real information on it, any idea what it could be?

You need to get this checked out. The most likely problem is air somewhere in the system, and your ABS system is going crazy dealing with it. The "Grinding" may be the ABS system cycling on and off at a very high rate… Making a very nasty noise. Once the system is "pressure bled" all your problems will go away… Like magic…