Holy cow! Nice inheritance! From experience I would rebuild all the brakes and the proportioning valve. You may have a collapsed hose, a flake of rust in the line or any other of fifty things that can screw up your braking. Replace all the lines and hoses too. It's not that expensive or hard to do. Do not part with any original parts from the car! If you buy rebuilt put the old in a box...etc. Also, buy the Ford shop manual for the 67 Mustang. The underpinnings are, for the most part, the same. Geez what a deal!