When I am stopped at a light and press my brake pedal, it will go down
to the floor.  It does this very slowly and will move down a bit
faster when I apply more pressure.  If I pump the brake up, it will
get hard and offer good resistance.  This will also occur sometimes
when braking hard.   This problem has been with me for a year and is
recently getting more noticeable.  I've bled my brakes several times.
There is no drop in brake fluid level.

I suspect that fluid is leaking from the master cylinder into the
brake fluid holder.  If this is the case, should I just replace the
master cylinder?  How long a job is this?

What's the real danger of  my current brake condition?  The van always
stops and offers good resistance on the brake pedal when I pump it.  I
intend to fix this, but I'm just deciding when to do it.

Thanks.

Regards,
Craig