Did you get the correct master cylinder? If the bore is too big the brakes will never work properly.
Have you adjusted the length of the push rod ?
There should be a return valve on the firewall which regulates how much brake fluid pressure is released from the rear brakes. It may need replacing.
Finally, adjust the rear shoes in the brakes until they lock the drum,then wind them off so you can hear them rubbing slightly when you turn the brake drums.