If your using the stock B body master cylinder and it was for drum brakes for both front and rear then you need to remove the residual valve from the line that the front brakes are hook up to, which is normally the rear line and reservoir
If it is the stock drum brake master cyliner remove the brake line and get a small # 6 or #8 sheet metal screw around one inch long and thread it into the brass insert inside that port, once it is theaded in two or three turns pry it out. The residual valve is behind the brass retainer, it looks like a rubber flapper valve with a small spring in it, remove it and reinstall the retainer and hook the brake line back up, make sure and tighten it real tight so the seal on the retainer will reseal in the master cylinder and on the brake line, bleed the brakes and let us know if that was the main problem or not