I've had multiple issues with the Cobra calipers on the rear of my Coronet. I've had to replace the calipers a couple of times when they fail to work properly. Not sure why I've had such bad luck with them, they seem to work fine in an OEM application. But when they get out of adjustment they will really reduce braking ability.
This is interesting. I somehow missed this when you first posted it.
A former Moparts member that is on FABO and FBBO mentioned that these Ford calipers have some weird quirks. Like what Sniper wrote, the parking brake ties into the caliper performance. I've read that the parking brake needs to be pressed and released several times to take up slack that forms as the pads wear. Failing to do so results in greater pedal travel before the brakes engage.
In addition to all of this, since 2001 I've used this '75 Dart single diaphragm brake booster:
