My 98 ford f150 has started doing something weird. If it was rained or just moist outside my brakes grab. I press the pedal and the grab real hard and makes a metal on metal noise. The pads are fine. If i power brake it for a minute it stops. It only does this like i said when it's moist out. To be clear its not from running through water. I'll go out start my truck and it does this only when i first get in or when it has sat for a couple of hours. doesn't do it if i'm driving and they get wet.Like i said it's weird. Any Ideas???????