What would cause this?

Jon March

on a "dual-circuit" master cylinder:

I had the following problem - maybe you can help me:

I bled the rears, and got firm pedal (as you probably know, the rears on an Amphi are "tee'd" from a single metal line fed by the first chamber of the dual master)

...then i went to the front passenger side wheel.
but when i bled it, rather than the pedal getting even higher and firmer, i lost ALL pedal firmness.... it went right to the floor.

Why would bleeding the front, cause even the isolated rear circuit (that HAD pressure) to lose ITS pressure??

I cant figure it out, but i bought another master cylinder in case somethings fucked up with this one.
Maybe its something else, but im stumped and thought i would try another brand of the same '68 style (recommended by a vw specialty shop)