Brakes

For the pass two weeks my brakes have been sticking on my 01 Sierra. When I apply the brakes the pedal will be stuck and it makes this humming/grinding sound and the truck doesn't want to stop. I believe it's the abs unit but I wouldn't know where to begin.

Stuck how, up, down, need more info?

The pedal doesn't remain stuck. It gets stuck as I apply the brakes but releases no problem. This doesn't happen all the time but daily.

I want to add that I changed brake pads 6-8 months ago then the rear brakes started locking down so I replaced the locked caliper. Two days later the other rear brake started locking down, replaced that caliper. No problems for 2-3 months then the same thing happened to the front brakes. Driver side locked down, next day passenger side locked down(bleed brakes everytime). Now I have 4 new(greased) calipers and that done good for bout a month and now this is happening.