Brake system problem

I have a 93 Ford Explorer 4 wheel drive with a rebuilt engine in it. The engine has 80k. I am experiencing problems with the life of my pads. The driver side brake pad which is located on the back side of the rotor is wearing out every 6 weeks. I saw a related problem on this site where you addressed the problem as being the master cylinder. I checked for leaks and low break fluid levels but everything was fine. How do I find my problem? Step by step if possible.