If you repeatedly pump the brake pedal when you're stopped, does it 'pump up' to the point that you finally get a high, firm pedal? If you release the pedal wait 15 seconds and then depress it does it go to the floor again? If you answered yes to both of these questions, you have air in the system and you have to bleed your brakes. It's as simple as that. If the answer to the first question is no, then you have a problem somewhere. I don't want to sound critical but IMHO, one of the worst things you can do, especially if you have ABS, is to just compress the caliper piston when you install new shoes. This just forces old brake fluid back into the system. I always open the bleeder and drain the old fluid when I compress the caliper.