New 944 Owner with a few questions

My brakes sometimes work great and are very tight(full pedal) and other times the pedal goes right to the floor and i have no brakes until i let the pedal back out and pump them agian. wut could this be? im guessing air in the lines but im not sure?

Sounds like a bad master cylinder to me.  If there was air in the lines, you would have no pedal all the time, not intermittently.  

ok thanks, u know what kinda price im lookin at for a new one? and about how hard it will be to install?

Depends on year and model, but plan on around $200-250 + brake fluid.  Not difficult to replace, but you will need to bleed the brakes as well.