Just lately, I have noticed that i can depress the brake pedal fully and hardly slow down. I have a newish (5 years) master cylinder that came w/ the disc brake and booster kit. Is this the prob? or is it due to air in the lines? It would seem that if it was due to air in the lines that the pedal wouldnt completely depress...or would it? Thanks

could be air but how did it get there
I would pobably say the master or a leak sounds more likely

I had that same problem before and it ended up being a bad master cylinder. But i was told it could be a booster that is getting fluid leaked in it.

Anyway hope this helps.