Hy 52 Cheese, did you order the booster/master cylinder kit for a drum drum system before you decided to do the disc brake conversion? Did you bench bleed both sides of your new master cylinder? Does your new system have a proportioning valve? Was a residual pressure valve for the rear brakes supplied in the kit? If you do have a proportining valve, check to see if there is continuity between the electrical connection terminal and the main metal body of the valve, if there is continuity, then the valve has been shuttled one way and is severely limitng flow to one half of the brake hydraulic system. In your case it sounds like the fronts are shut off.