I am converting a 65 C-Body from the single reservoir master Cylinder and Drum brakes to a Dual Master and Front Disc Brakes. The brakes are still manual brakes and everything is done, but I am having trouble getting the pedal to come up and I am not confident I am getting enough fluid distribution. I used the original brake push rod from the single master cylinder with the dual, the pedal travel seems right, does anyone know if there is any difference in length? I am using a new disc/drum master cylinder, but the brakes just dont feel quite right, the pedal will travel to the floor and return to near normal, but it seems like I have a little slop at the top position. Any Ideas?

Did you use a master for non-power brakes? I changed my '64 over to discs and used an A-body manual disc master. I'm pretty sure I used the old push rod. My pedal is good, doesn't go to the floor.

there is one thing I can think of, assuming your car stops fine.
the retaining rubber on the push rod, and if there is a grove cut in the master cylinder for it.
if that is missing, I think you would experience what you are describing.