2002 Pontiac Grand Am Fast Pad Wear

I replaced the front brake pads and rotors with middle of the road pads. They had a lifetime replacment warrenty. 6 months later those pads were almost metal to metal. I replaced them again, the auto store said the calipers were bad. I disagree because it was both sides that went bad. When I replaced them I noticed that the brakes were spongy and the car didn't stop like it did before. I had to push harder. I also noticed that they didn't release quickly. Now a couple more months later the rear brakes are making a grinding sound. I have not bleed the brakes but I don't think it will help. I believe the problem could be in the Master cylinder or the ABS. What could it be and how would I test this.