Slop and "woosh" sound with brake pedal?

[boy&hisdogs]

So I just swapped to a NBS master cylinder, but this has been happening forever, both while driving and with the truck off. Once I actually get into the brakes its fine but for the first little bit of pedal travel there's a wooshing air sound and some spongey slop. What could be causing this? Im sure there isn't any more air in the lines, my dad and I bled it 3 times. The pedal does feel firmer and more effort required to push it, but I'm not sure if thats the new m/c or a bad booster or just power of suggestion since it's not very much.

What could be causing this sound/slop and how can I fix it?