1997 GMC Sierra Brakes Shaking Truck

When I use my brakes, my truck shakes violently. When I first start out in the morning, the first couple of times I use my brakes there is little or no shaking, after that it comes back with a vengence. I replaced the front brake pads, calipers and wheel bearings. I also had the rotors turned (they told me the rotors were in good shape and needed very little turning). Everything got better for about a month and then the shaking came back. What else could cause this or what could I be doing wrong? Air in the lines? Master cylinder? I am at a loss. Also the rear brakes look good and are adjusted properly.