Psssssst, small hint here, if you know that your cooling system has any
leaks ANYWHERE (rear heater core included) you will never get your cooling
system properly blead (sp).

The water when it cools will contract (physics remember) and when it
contracts it will draw air in where the leak is, when you start the car the
water pump pushes the air bubble to the highest point, typically your
radiator...  I had tons of problems (could hear the bubbles in the front
heater core in the morning) before I removed the leaky rear heater core.  I
removed the rear heater core and replaced it with two elbows and a piece of
hose till I can find a non-leaky one.

BTW my leaky rear heater core was also leaking from the core.  I tore the
whole sheebang apart and the rubber on the ends (not the hard plastic) that
seals the individual pipes has disintigrated (you can scrape it away with
your fingernail).

Dean