New BAP brand MC that I bled on the booster with plastic fittings. Also replaced all lines in the firewall including distribution block. And new ss braided -3AN at the front calipers.

I have bled the **** outta it and still it feels weak then grabs late. Still stops but something is mushy. Pedal is the right height per FSM.

Booster seems to work because upon startup with the pedal depressed it goes down.

 

The easy answer is that there is still air in the system.
It's easy because that would be expected after the extensive amount of work you've done and the number of points you've breached in the loop.

You will probably come back with "No, I've bench bled it and pumped till no more air ever comes out!", so let's look at other options.
Both sides of the booster are adjustable, pedal>booster and booster>MC, one or both may need tweaking.
Have you carefully checked all the new connections you made?

Your "new" (probably rebuilt) MC may be bad...it happens.

And btw, there is only one distribution block in a non-ABS 7 and that's in the back.
You were referring to the proportioning valve.

 

No, I've bench bled it and pumped till no more air ever comes out!

Yea the proportioning valve. I can't believe how there could be air, I mean that's what I thought it was but I bled 2 huge bottles of fluid through it.

The MC could be bad. It sucks because it only had about 500 miles on the rebuild and that much on the MC but because I installed it a while ago it's not warranted.

I also thought that maybe they have me the 4 lug master cylinder and that's why it feels weak

 

It's the "feels weak" part that makes it hard to diagnose from afar.

A "small" MC needs less effort over longer pedal travel, a "large" MC just the opposite, less travel, more effort.
But both systems are actually braking throughout the pedal travel distance, it's only the "feel" that changes.

When the pushrods are improperly adjusted, that's when you get the "dead pedal" that only engages the brakes at the extreme of the pedal stroke, your input at the pedal is not being translated into MC piston travel.

You say the pedal height has been adjusted but that's a deceptive claim...the pedal height is a purely mechanical adjustment of the pedal itself and has no effect on the pedal>MC, which is completely dependent on the two pushrods (after the static height has been set).

When you push the brake pedal by hand, how far does it move before you feel resistance?
You need some freeplay but only a bit, not more than 1/2" (with the brakes cold).
If you have more, try adjusting the booster>pedal pushrod to take out the slack and see what happens.
Remember, you need some freeplay...I like to do the final adjustment when the brakes are at normal temp (the expansion of the fluid is what eats up the freeplay) and leave about 1/4" of slack.

That's the easy adjustment and I hope it works.
If not, you might explore the adjustment between the booster and the MC but that's much trickier to pull off.
Too much preload on these adjustments will cause the brakes to drag or lock up, so be careful.