Just a follow up. The brake pedal not returning had continued for the past 5 months so I decided to contact SSBC and have them send me a replacement vacuum booster and master cylinder. I never felt anyone else could drive the car while having to use your left foot to pop up the pedal after each braking, but otherwise the brakes felt normal.
Now I'm having the same problem that so many others have documented about our vintage car brakes.
The new parts are installed and I have about 2-1/2" of dead pedal at top and then the pedal is solid and stops the car if you push hard enough without the feeling of the power booster assist. SSBC says it HAS to be air in the lines and not a faulty MC. I've bench bleed the MC and gravity bled the brakes as SSBC recommended 3 times, then tried to vacuum bleed the system and finally paid for a pressure bleed in desperation. The results have had no change. I also make sure the booster rod and MC had a .000 - .015" clearance and that the rear drum brakes were adjusted. All those who have documented this problem on blog sites never posted a solution, just theories. Many bloggers said they installed an entire SSBC brake kit and still had this problem using all new parts. I've got the MC out of the car and sitting in the vise for another bench bleed wondering what to do next. So sad I can't use the car until I can find a solution. If you have ideas, I'm open to try something different. I'm thinking about replacing all the rubber brake lines and the proportioning valve just because I don't know anything better to try.
One note, I was always unhappy with the gravity bleed of the rear brakes. They took as long as an hour to bleed an ounce from each wheel eventually stopping. I don't understand why.
1966 Mustang 2+2, A code, AOD AT, PS, PDB, AC, Pony deluxe interior