Nissan used a dual point distributor in the 1970's for controlling emissions. The second set of points was retarded 5 distributor degrees, or 10 crankshaft degrees, in third gear, part throttle. The second set of points is not used any other time. The second set of points is connected in parallel with the first set of points, by a relay. There is are switches on the transmission, the clutch pedal, the throttle pedal, and the carburetor, that control the point relay.
521 wheel cylinders only have one port for fluid to go in and out. You have to use the motion of the brake shoes collapsing when pressure is released to push air out of the wheel cylinders, and out the bleeder valve. Using a pressure bleeder does not allow wheel cylinder air to escape the wheel cylinders, and using a vacuum bleeder on the wheel cylinder bleeder probably will not remove all the air from the wheel cylinder.
You have to bleed 521 brakes the old fashioned way, with two people usually. Pump up the brakes, and hold pressure. While pedal pressure is being held, open the bleeder valve, and keep it open until the brake pedal goes to the floor, and the shoes collapse again. Then close the bleeder, and after the bleeder is closed, then the brake pedal can be allowed to come up, and pumped up again. Three full movements of the brake pedal, and refill the master cylinder.