91 Bonneville front brake problem

The front brakes have been replaced twice in the last year and a half and they will have to be done again soon. I only average around 10,000 to 15,000 between brake jobs. Both times in the past I also replaced the rotors and the calipers. I'm thinking maybe the master cylinder is causing them to wear prematurely but there is no dragging or pulling. If anyone has any ideas or suggestions it would be appreciated.