You've got a problem somewhere in the hydraulic part of the system:
1) Bad master cylinder again.
2) Air in the system still. It may be in a high spot where it's hard to move. May need more pedal pressure if bleeding traditionally or using pressure or gravity bleeding. I assume the new master cylinders are properly bench bled, either on or off the van
3) Some off mechanical problem like a flex hose swelling outward, too much wheel bearing slop on a disc brake. rear drums way too loose, etc.
Here's the best test (from a Mopar Action article):
Remove one of the hard lines to the master cylinder and block that port off with a bolt. Did the pedal get firm? If so the problem is in that circuit, I don't know if these are one circuit front and rear or diagonal.
If the pedal is still bad, remove the other hard line and plug it. Did the pedal get firm? If so the problem is that circuit.
If the pedal is still not firm, it's the master cylinder.