This is driving me nuts. I had the MC go out and got the pedal to the floor etc. Now just replaced the MC and bled the brakes. It will get pressure and firm pedal with the engine off but once I start the engine the pedal goes straight to the floor again with no pressure. Also getting a slight whoosh when pressing the pedal with the engine on. I guess the booster may have gone out as well since there was some brake fluid in it. But shouldn't that make the pedal harder to push not easier?