soggy brake

i just replaced all rear lines on my light truck cuz they rusted out.
now, here's what happens: when the engine is off or the booster is disattached from its vacuum feed, the pedal gets solid after 3 or 4 pumps. as soon as i plug in the booster, the pedal gets soggy and lower.
however, the brakes still work fine.
does this mean that:
1. there's still air in the lines (I did bleed them), and it only gets compressed by the booster
or
2. I pressed the pedal too far while bleeding and damaged the seals in the master cylinder
or
3. the booster itself is leaking

when applied, a hissing sound comes from somewhere around the booster and mstr cylinder, but I can't tell exactly where.
any ideas?
thanks
oh yeah and I don't think it was like this at all before, but I just bought it so I'm not positive

yo
sorry for redundancy
i'm new here and didn't see previous brake thread
still, my question kinda stands cause I don't understand how the booster ties into all this

The booster makes it easier to press the pedal.
A hissing sound is probably just the vacuum line not on the brake booster all the way...
Solution to your problem?
That's the way it's supposed to be...no problem at all...
BTW, what kind of light truck to you have?