Brake Disc Pads problems of the 1998 Chevrolet K2500

The abs brakes on my truck have never worked correctly and, if you brake hard and hit a little bump in the road at the same time, the brakes cut loose and don't stop. An accident occurred in 2000 due to the brakes not working. When you hit them hard, they cut out. Complained numerous times to 3 different Chevrolet dealers, and was told that that was a normal brake functioning. Have changed brake pads several times and brake rotors and hasn't helped a bit. Truck is scary to drive. Only have 56000 miles on it due to trepidation in using it. Have had nothing but problems with it since the day I bought it brand new.