Brakes won't hold pressure

Hey everyone, I just swapped to 1973 single piston front disks and 10 in rear drums. The car used to have 9 in drums on all four corners. All brake lines, hoses, calipers, wheel cylinders, and correct disk/drum MC are either brand new or 2 years old at most. Only thing that isn't new is the old drum/drum distribution box (never had issues with the dist. Box with the old drum setup). Have tried multiple master cylinders with no change In results. I have a wilwood adjustable prop. Valve plumbed into the rear brake line. Bleeder screws are on top.

Now for the problem, I can't seem to get the brakes to bleed. If I pump the brake peddle around 4 times the peddle is firm and doesn't seem to decrease if I hold it. But if I let the peddle return, wait for a few seconds, and try to press the brake pedal again there is no pressure behind it and it goes to the floor. I have gravity bled, pressure bled (by pumping/holding the brake peddle), and used a vacuum bleeder on the system starting from the furthest point and working my way in many times. Always seemed to have good pressure/no air bubbles coming from fluid when I would crack the bleeder screw. All connections don't appear to have any leaks.

Could it be the old dist. Box causing this? Figured I'd ask here before I spend anymore money. My apologies if this has been answered before, I couldn't find a solution through all the treads I read that I haven't already tried. I'm trying not to get frustrated with this car but it's starting to get hard