>I am replacing the single circuit with a dual and have found the following
>problem.
>
>After bleeding the fronts only, my brake pedal goes right to the floor.

YIKES!!!  No biggie...  You have to bleed ALL of the brake lines after
replacing the master cylinder. 

>ps: I haven't bled the rears yet due to a leak.

Thar you go - fix that leak.  You have to, or you will never have brakes again!
Never ever ever again :-)

Brake bleeding procedure: 

bleed all four lines starting with the rear wheel that is farthest away
from the master cylinder, i.e., on the side of the bus that is opposite
from the master cylinder.  

Then bleed the other rear wheel cylinder.

Move to the front, and bleed the line to the wheel that is farthest away
from the master cylinder.

Then bleed the other front, and last, brake line.

Test the brakes, and repeat until it is finished.

HOT TIP - get one of those one-man/one-woman brake line bleeding hoses, the
kind with a clear plastic hose and a little check valve on the end.  I
always thought that these things were way too expensive (I paid $10) for
the little bit of plastic and metal that you get, but since it made my job
SO MUCH easier, I am very happy that I have one, and now bleeding brakes is
a job I don't procrastinate with.

Wolf