I have 32,500 miles on my Stallion #538.
I pulled the left rear wheel off and checked the brake pads, the outside pad was gone so I had metal to metal.
I bought new pads for 2006 ford five hundred and they are the same kind.
now I ran into a problem trying to get the piston back into the caliper, a "C" clamp while NOT do the job. I found out that you get to buy a tool that will allow you to SCREW the piston back down, I got the tool from O'Reilly auto parts store, it is a cube with a hole in the center of each side that a 3/8" socket wrench fits into. each side has a different pattern on it to fit the possible different patterns on the part of the piston that is against the pad next to it.
The reason that you have to screw the piston back down is that the emergency brake cable is attached at the back side of the caliper assy and every time you put on the emergency brake you mechanically screw the piston out, if you hit the regular brake pedal you hydraulically push the piston out, if you do it with the emergency brake it will mechanically go out and WILL NOT go back in when you release the emergency brake. that is why you need the tool to put the piston back into the caliper.
the tool came from a company called "LISLE CORP" and they call it a "Disc Brake Piston Tool" there part number on the package is "28600". I'm sure there are other manufactures of a like tool.
O'Reilly calls it an "adjuster". O'Reilly sells it for $14.06
I then checked the other rear brake caliper on the other wheel, the same problem in that the outside pad was worn a bunch and the inside pad was hardly used at all. I used the tool to put the piston back in and put on new pads and all is OK.
I would check the rear brake pads on both sides at 20,000miles to see if they are wearing evenly, if not all the pads are the same so just reverse the pads on the caliper and then do the same on the other wheel.
I WOULD NOT USE THE EMERGENCY BRAKE UNLESS IT IS A REAL EMERGENCY SO THE PISTON COULD BE PUSHED BACK EASILY WITH A "C" CLAMP
another bad problem with using the emergency brake and the piston being mechanically pushed out is that it is VERY difficult to remove the caliper to replace or swap the brake pads.
it is up to you if you want to use the emergency brake as a parking brake or what ever but I will NOT use mine unless the hydraulic brakes go out while driving.
another GOOD tool to have can be gotten at WWW.motionpro.com, it is called "Hydraulic Brake Bleeder"
and "Easy Bleed Tool". with this you can bleed the brakes by yourself and do NOT need a second person to help out. the part number motionpro uses is 08-0143, when I got it it sold for $20.95