I just replaced the front pads and rotors on my 98 F150 last fall. This week I noticed a grinding noise so I took them apart to investigate. I found that the left outside pad was completely gone, but the other pads are at about 75%. Why?

 

Sounds like the caliper is not releasing properly. Either the slides are sticking or the caliper is bad.

 

yep... search for postings about how to clean and grease the pins...