Just to point out, there's a few things that might prevent fully travel. Linkage binding or hitting something, the booster might not have correct travel, and even the master cylinder can bottom out before reaching high enough pressure.
Disregarding pedal pressure, has this been run as a manual setup? Were the brakes able to lock up then?
Note, the Mustang calipers are different from the Explorer ones. No parking brake in those.
Interesting, my Mustang had parking brakes in the rear caliper, guess it depends on exactly what year Mustang we are talking about. Blowing up the OP's picture, it sure looks like there is a spring there for the parking brake lever. In fact, those calipers look just like the ones on my 96, which most assuredly did have the parking brake built into the caliper.
I don't see any actuating cable for it though,
