Victor,

Karl had a good suggestion, but the usual failure mode of brake hoses is
catastrophic since they are re-enforced against stretching.  But still ...

Your problem description isn't quite clear to me.  If you put the minimum
amount of force on the brake pedal to cause the problem:

1) Does the pedal slowly sink to the floor, or

2) Does the pedal only go further toward the floor when you add more
   pressure?  In other words act something like a spring.

If number 1 is true, then fluid is leaking somewhere; either past the
pistons in the master cylinder, or to the outside world from someplace.

If number 2 is correct, you have air in the system or something like Karl's
suggestion is happening.  Since I've never seen stretchy hoses (not that it
couldn't happen) I would go with the air theory.  And, even if you bled
your brakes, air can still be re-entering the system through the fault just
as fast as you bleed it out.

I don't see any indication of a brake booster problem.

So, is it #1 or #2?

Mike '84 GL  '58 Isetta   Phoenix