brake problem

[jaybyrd66]

ok guys, got a question for ya, hopefully i can pick at someones brain for a lil bit and come up with some kind of solution here. ive got a 2006 silverado 1500 2wd with 4.3 and auto trans with avalanche 20s on it. heres my problem. it started a couple months ago and has got worse and more frequent here lately when im at speed everything works just fine but when im rolling real slow like thru a parking lot or down my driveway and hit the brakes to stop and at almost every red light and stop sign i come too i hear this kinda grinding noise like my ABS is trying to kick in and then my pedal will drop about an inch and my front brakes just let loose and all i have is rear brakes, i know this cuz i checked in my driveway and when they did it i pressed real hard on the pedal and the back tires just slid and the front didnt do a thing. but i can let up and press the pedal again and everything works fine again. then it will happen again about every time i stop. any ideas at all are appreciated greatly.