Did you bench bleed the master cylinder before you installed it? You have to do that first or you'll be bleeding the system forever. Don't forget to when you bleed the entire brake system, to start at the farthest wheel from the master cylinder and move in closer as you go. IE: Right rear wheel first, then left rear, then onto the right front and lastly the left front. You opened the system to installed the master cylinder, you'll need to bleed the entire system.