Unusual Squeal

I have a 1992 Buick Century. Sometimes, when I apply the brakes, I hear a squealing noise coming from the right front side. I hear it mostly when I have to stop abruptly at slow speed. But, it’s no ordinary squeal. It sounds like a rubber balloon being rubbed. Most important, the noise sometimes continues for about two seconds AFTER I come to a complete stop, Could there be a problem with the hydraulics? Does anybody have any ideas?