<VV> Brake Question

[Vair Chris]

My 65 Monza convert has the dual master cylinders and disc brake upgrades.
I am using silicon brake fluid.  The car always stops great and the pedal is
hard.  However, when I put it in reverse and put on the brakes, the pedal
gets soft, and if I stop more than once in reverse, it will go to the floor.
When I put it in forward again, I can pump the brakes up and they will be
hard again.  This seems like odd behavior to me.  Thoughts?