After replacing both brake lines to rear the brake pedal goes to the floor when starting the car?

It had a leak in a brake line. Replaced both lines to rear. Bled system. I had to replace the master cylinder. Bled lines using vacuum pump. When the car is off, the brake pedal is high and solid. When car is started pedal goes to floor. No visible signs of leaks under the car. When bleeding front drivers' side, had large air bubbles with fluid coming through.