The front heater core is essentially self-bleeding. When the engine is running cold, the waterpumps entire output is moving thru only the heater hoses, because these hoses are not stopped by the thermostat. Since the waterpump has ALOT more pumping capacity than it needs to run just the heaters, the flow is effectively much greater. If you close the rear heater, but leave the front open to full hot, all of the flow that the waterpump has available, at least while the engine is cold, and the thermostat is closed, will be going thru the heater core. As long as there is enough pressure developed to overcome the height of the heater core above the pump, you will get flow into, and therefore out of, the front core. There may be bubbles that take a while to cycle out, but every time you start the engine cold, a bit more of the air that may be trapped will be pushed out.

Larry Alofs <lalofs@GMAIL.COM> wrote:  Now that winter has arrived in Chicago, I have become motivated to get the
heaters working in my recent Subie conversion. This has involved draining,
refilling and bleeding the cooling system a few times as I try different
configurations.
The radiator has a bleed screw, the rear heater has a bleed screw, but the
front heater does not. I don't understand how the air in the front heater
is ever supposed to get out; it is considerably higher than the heater hoses
which run under the van. It seems to me that if there is any significant
air in the front heater core it would not transfer heat very well and the
coolant would not circulate the way it should.
Does anyone have an answer, solution, or some reassurance for me?

Larry A.
'91 GL with EJ22