angela is right, it sound's like the master cyl. is going bad. that's how they start going out, when you stop and then back off of the pedal just a lil bit, they'll let the pedal slowly go down to the floor.
also, you said you changed the brake fluid, so that tells us that the brakes are already bled. you cant change the fluid without bleeding it out!
oh and you're better off not going to a brake place for this problem, because they're gonna bend you over with no KY !!!!! they'll sell you everything in the brake system!!! just get with someone to help you with it cause it's an easy fix!