Converted my GTX over to 73-76 Dart/Duster disc brakes this weekend and the brake pedal seems like weak sauce. Rebuilt Autozone calipers, Wagner Thermoquiet semi-metalic pads, new rotors and new Wagner brake hoses.

Power bled the calipers with a electric vacuum pump and the old fashion way with an assistant. Clear bleeder hose shows nice clean fluid with no bubbles. Calipers are pointing the correct direction (bleeders UP)

Car has a '74 Disc brake master cylinder with no booster which was on the car when I was running the 11" drums and works fine. If I turn the line lock on and then hit the pedal its iron. If I heave on the pedal with the line lock off the pedal will go 5 or 6 inches down.

Driving the car is a pleasure, I can hit the brakes and the car doesn't dive about like it did with the old drum setup. The car doesn't lock up the brakes but I can stop in a hurry if I bury the pedal as hard as I can.

Admittedly, I only have about 40 miles on the brakes and perhaps the pads still need to be fully burnished in. I'll continue to drive the car and work the pads in but it seems to me even so the pedal could be better. I ran my Dart with the same setup decades ago and don't recall the pedal being this soft with a manual disc setup.

Inline Tube makes braided hoses for the front, good investment? For $60 shipped for the pair chump change that I don't mind spending.

I'm 100% positive there is no air in the system, even went as far is to put 26 inches of vacuum on the caliper bleeders and pull a half quart of fluid out on each side.

And then manual bled them with an assistant for good measure.

The rears and master cylinder has already been eliminated as the issue, the line lock is on the front circuit after the master cylinder.

when it acts up it will not pump up at all?

The braided hoses help firm pedal feel a little. I like and use them. That said, I think you have more sponginess than can be explained by the hoses unless you have a weak or defective hose (which may be). 5-6" of pedal travel is a lot of difference line lock on and off.

did you put a proportioning valve in place of the distribition block ?

Like I said, its possible the pads are not bedded in. Maybe the compound is a bad match for the car without a brake booster. The rotors are new '78 big 11.75" rotors with the Duster slider calipers, one would have thought it will stop better. It did improve somewhat after a long roadtest with lots of heavy braking with cool down between applications.

P.S. the Master cylinder is the 1-1/32" unit if curious.

I suggest you look into what the factory combination valve does before you dismiss having one or think just putting an aftermarket adjustable proportioning valve in is the same.

Make sure none of the pistons in the calipers are retracting to far after the pedal is released.
Had a brand new one do that. Turned out the seal bad and was pulling the piston back 1/8".

Is it possible the 1 1/32" MC does not provide the volume of fluid needed to make the calipers more aggressive?? You said it used to dive with the drums, which to me means it had substantial front brake. Calipers do require more fluid volume than a wheel cylinder don't they?
My 70 Satellite has the same issue. It has a long pedal on the first pump but it stops fine. Its been bled and bled with no difference. Makes my think maybe the M/C bore is the culprit. The line lock comment makes sense when mine is on and you touch the pedal there is nothing but hard line in the system and no way for the pedal to move whatsoever. This is normal.