1996 Chevy Astro AWD 212,000 miles 4.3 V6

This vehicle was purchased used two years ago. It has nearly always had it as long as we have owned it.
The problem centers on a very hard "pull" to the left when braking. All wheel braking hardware has been replaced, (calipers, rotors, drums, cylinders, etc.) And have been bled numerous times causing no change in response. The pull is greatest in the initial brake pedal pressure and is worse when braking hard. This pull will completely turn the vehicle. There are no obvious problems with brake lines. The pull to the left is sometimes followed by a pull to the right as well as the a feeling that the front left brake sometimes requires 3-5 seconds to release. All mechanics thus far have no idea other than to change Master Cyl, ABS and Dump valve at a very high price. Can you offer any direction for where to start?