the Expy and F-150 share the same platform...and at least 75% or better of the parts interchange
on the F-150, the brakes are a 60/40 split (front/rear).
people are always surprised at how quickly their brakes wear out in the rear
it is very likely the Expy is the exact same setup
I know that doesn't exactly answer your question, but it's a bit of an explanation.
also, does your Expy employ the RSC on your pretty often?
this might be the case as well. especially if you are an aggressive driver, and take corners a bit fast, or accellerate quickly
the RSC uses the brakes...and maybe it's hitting the rears alot