Do you run the engine when your filling it with coolant? If not jack up the back of the car just a little bit with a jack and jack stand, not too high just enough. Open the radiator cap and breather valve on the radiator and go start your Z. Let it idle till the thermostat opens and start adding fluid. The air pocket could be stuck somewhere so the angle and fluid will help push it out.
If at any point your coolant level drops than that was your bubble. Cap it up. Put it down on the ground. See how it works.
