Power Disc Brakes not working on hard stop?

Just saying
My 68 Barracuda has the KH 4-pistont calipers and rotors, that I pirated from a 70 Duster. I swapped the Dual Diaphragm out for a 73>75 Single, I think it camo from an F-body. The M/C was pulled from a mid 70s car. The Combination valve was pulled from a box of used stuff I had, and reconditioned.
The difference in the Early to late M/Cs, AFAIK, seems to have just been a reservoir sizing thing; I say because I have used either, with equal success.

The point is this:
IMO, when it comes to brakes, from 67 to at least 76, just about anything goes.

Post 77 edited at the bottom.