I know the stopping power of a power disc brake '71GTX is much better than what I'm experiencing with my car. In fact, it makes me nervous, but I can't seem to isolate the problem. Here are the specifics:

1. Restored Midland-Ross booster with lever assembly & short reinforcement plate(Booster Dewey).
2. Correct '71 power disc "snout nose" type remanufactured Master Cylinder (Cardone).
3. All new stainless brake lines, hoses and proportioning valve (Inline Tube).
4. New Rotors & rebuilt calipers.
5. New riveted semi-metallic disc pads, rear linings and wheel cylinders.
6. Intake Manifold vacuum at idle: 11"-13".
7. DOT4 brake fluid.

When applying the brakes, the pedal moves about an inch or two, but never to the floor, even when applying greater foot pressure. However, braking response remains inadequate. I can't say that I would consider the pedal feel as excessively spongy. My brother has the same car and I know he can lock the front brakes...mine never comes close to doing that.

I had the brakes checked for air in the lines and the rear brakes adjusted. Someone mentioned that the rotors may be glazed, but I took a peek at them and didn't notice anything unusual. There are no obvious leaks.

My research as to some possible causes are as follows:

1. 1971 Chrysler Chassis Service Manual shows power booster with lever assembly for 10" rear drum brakes (p5-27), and booster for 11" brakes (p5-28). I have the setup for 10" brakes, yet I have 11" rear drums.
2. Incorrect Booster Pin length (or needs adjustment). I don't have a clue on this one.
3. No residual valve in master cylinder for the rear drums. Don't know this one either.

I am hoping that someone could help me resolve this so that I can fully enjoy driving the car. Thanks for any input.

adjusting the pin that extends from the booster into the master cylinder may help. the service manual will give details. i've done it before its easy.

Do I have to remove the master cylinder from the booster? How much adjustment is adequate?

yes it has to come off.

I find I have to play with it to get the right adjustment.

carefully, with fluid in the master, or empty it to be safe,
look in the bottom and you will see a hole in the resvior for the fluid to get to the piston.

As you push the master back onto the booster, you can see if the piston moves.

if it moves, too long. I extend it till I see it move, and then back it off a little till the piston does not move when I push it onto the booster.

Check the rod free play as Andrewh said & w the lack of travel that suggests to me either a major blockage in the prop valve (not likely) or a booster internal prob. Holler what it ends up being

Another thing to test is if the booster is actually working, Booster Dewey does nice work but things can happen.... With the engine not running pump the brake pedal a few times till the pedal is high & firm with no vacuum stored in the booster... Start the engine... Does the pedal fall slightly? If not the booster isn't working....