Why dont I have any pressure behind my brake pedal?

I replaced my front brake pads and rotors on my 1999 Saturn Coupe. I bled the brakes several times until there was not a single air bubble coming out. The brake pedal will be extremely hard after bleeding them but only when the engine is off. When I start the engine the brake pedal goes to the floor and there is no pressure behind the pedal at all. I took it driving and tested the brakes and there is a fairly large amount of lag behind the brake pedal, in other words I have to hold the brakes for a while for the vehicle to come to a complete stop. There is no visible leaks from any part of the brake caliper. Does anyone know whats going on?