Front Brake Caliper Problem

I have a 96 Previa SC with 4 wheel disc brakes (2WD). We’ve had brake problems over the past 2 years with it wearing out front pads very quickly (like 10-15k miles), and my wife doesn’t ride the brakes while driving. Our mechanic tested brake line pressure, and discovered that the proportioning valve needed adjustment. This didn’t solve the problem. We then installed new calipers, and still had a problem with the left front (right front was OK). Rear brakes are wearing fine. My mechanic has covered all of the repairs under warranty, but says that he cannot figure out why this is happening, unless he got a bad caliper on the LHS only. But, he said that it appeared to be releasing fine on the bench. Could it be something with the ABS system (although we aren’t getting any warning lights on the dash)? Any ideas?

probably you can try replacing the disc. also make sure that the small wire pieces that insert into the brake pads are not bent or broken.