1982 Mercedes Benz 500sel keep getting air in break lines

I replaced the break pads on my merc (1982 500sel) and have bled breaks about 7or8 times. Breaks work ok for short time but then fail, pedal goes to floor. When pedal pumped here sucking noise comeing from behind fluid resivore. When I was replaceing pads resivore did run dry. ABS warning light is lit up