Brakes don't work after hours of not using

When I haven't used my car for a couple hours (overnight, 8 hour work day) and I try using my brakes they don't work. I can push the pedal all the way to the floor and it will slowly stop. I try to pump a few times before I go and that normally seems to help. By the time I am out of a parking lot they work fine and no problems at all when driving around. I have had a few conversations with co workers and friends that have mentioned the brake line or master cylinder. I plan on taking it in but also wondered if anyone knows the approximate costs for these repairs or if these are even good explanations for my problem.