GLC Brake Problem / Need Advice

My 1985 Mazda GLC LX had corroded brake lines connecting to the rear brakes. I replaced the lines and went through brake bleeding procedures. There appears to be no leaks and no fluid loss. There is resistance when the brake pedal is depressed when the engine is not running. Once the engine starts the resistance in the brake pedal disappears and the pedal goes to the floor. I assume that there may be a problem with the vaccum boost system. There was no problem with the system prior to the brake line rupturing. Since the bleeding process went normally, I have no idea what to do next. Does anyone have any insights?

HELP!

Thank you in advance.