Question about bleeding brakes

[western star]

71 with a dual master cylinder. I replaced the brake line that runs form the junction block on the rear end to the drivers side rear. I did have someone pumping the brakes and holding pressure on the brake pedal. The first time I cracked the bleeder I got air. From that point on all I get is a trickle of fluid. After trying this for quite awhile a someone suggested that I might have a saftey valve in the brake lines that prevents all brakes from failing in case of a bad leak therefore I'm getting mostly front brakes only right now. All you have to do is reset it. Great but I have found no valve but there is what apperars to be a button on the master cylinder with a rubber boot on it. . I thought I would ask before proceeding. Any help would be appreciated.