I have a friend's 68Charger. Has four wheel disc brakes. They are manual due to the engine with a blower. Have tried three different bored sized master cylinders. Last one is a Wilwood. Car will stop but not well at all. I am brand new on this forum thing, so bear with me while I learn. Thanks

 

Sounds like the system was spected undersize? But lets just assume the caliper pressure is good..
What we have found if most everything is in good order, it that the brake pad itself will be found at fault !
We called it a squishy pad.... Pad collapse when the caliper tries to squeeze the brake pad...
Maybe checkout a set of Performance Friction Carbon Metallic pads. See if that helps the issue..

https://pfcbrakes.com/

 

I agree with checking the pads. Pads collapsing is a real issue causing a soft spongey pedal.

 

Go over every single connection I suspect you have at least one drawing air , however you could have a bad caliper. Are all the calipers installed correctly, bleeder up?