Brake Problem

It takes my pressing of the brake to the floor to slow up my car. I looked up the problem in my Haynes book. I checked to see if it was the power brake booster and apparently it's not. By what the book stated, the air is tight and not the problem. So...it says it may also be pads, but this problem just all the sudden happened? Can pads just go like that? Maybe a brake cleaner can help? I have an appt tomorrow morning. Ah, it also said the brake lines, which is my guess, 'cuz a few times the brakes worked fine, making me think the brake fluid is getting there. I filled te reservoir up so that isn't it. Any ideas?