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.

If I pump a few times in relatively quick succession, I get (subjectively)
braking power.  At this point, if I hold the brake pedal down as far as it will
go (it won't go to the floor anymore), it won't slowly go to the floor
which I take to mean that I don't have a leak.

Question:  why is the pedal going to the floor?

...Markus "the puzzled brake novice" Benne

ps: I haven't bled the rears yet due to a leak.
pps: No, I'm not driving the bus right now.