no front brakes on 71 beetle

have replaced master cylinder 3 times and still cannot get front brakes to work. back brakes bleed fine, but front brakes will not work even though there is no air in the lines. bench bled dual MC and ran more than a quart of brake fluid through with no air bubbles but still no brakes on front. have worked on austin-healeys for more than 40 years and never had this kind of situation, although will admit to first time changing out a dual cylinder MC. have adjusted brakes up pretty snug. no leaks and flex hoses look fine. any ideas?