ABS brake problems?

My Van with ABS has had brake problems for two years and over the course of these two years I've had two complete brake jobs, new Calibers on the front, had the drums and disc turned, bleed the brakes and the problem is that whenever you perform any of these mention things to the brake the brakes would work fine, then 2-4 months later the brakes would become very touchy whenever you step on the brake pedal at a slow speed it would just about put you though the windshield until I get the brakes bleed or get a brake job etc. Can you please help? The Chevy dealer doesn't know what would cause this problem so they are recommending a complete replacement of the brake system for $1500.00. I am sure it has got to be something simple.