So, where can I get one?
Preferably, something I can get from the corner parts shop.

69 barracuda, '73 front dart discs, '84 caddy seville rear discs

Dr. Diff was working on one don't know if it ever came out???

I used a manual master cylinder from an early 80s truck. It's WAY too big for the M-body calipers in front and Caddy rear calipers.
Hard braking usually required firm application of both size 15 feets and 225 lbs of my chunky butt hovering above the seat.

I put the monster AMG calipers on the front and now the pedal feels just right.

I don't think you'll ever find a master cylinder that will work with A body calipers up front and Caddy calipers in the rear. Those two items are so mis-matched that you can't solve it with a single MC. You would most likely need dual masters to make that combination work with the MC feeding the rears having a much smaller piston.

I use an early 80's power brake master with and adjustable valve for the back. Stock a-body up front and caddy in the back. Stops great. It's the aluminum bodied master with plastic resivoirs.
RAYBESTOS PT#MC39178

Andy -

Would an adjustable proportioning valve solve the problem with the rear brakes? I bought one of the TSM kits a long time ago and planned on using it and the 11 3/4 Cordoba brakes (from your stock of rotor club for men rotors ) on the front of my '73 Challenger. Seemed like it would be good upgrade?

Can you expand on why one of the truck masters won't work? I'm a little too far into polishing one of these master cylinders to believe that it wont work. But I would rather be safe than sorry...