If you don't turn the temperature control to max to allow coolant to flow through the heater circuit, it may take quite some time for the entrapped air to purge. If you do the job in the early summer air could be entrapped in the heater core circuit until the HVAC system determines that heat is required and opens the valve. This could take MONTHS, and any entrapped air can rapidly increase corrosion in the affected area, which could cause premature failure of the heater core.

I have never removed my thermostat housing to refill the coolant or found it necessary to install a "bleed valve". It's totally unecessary if you slowly fill the system through the cap opening and allow it to self bleed with the temperature control on max.

I think on the the biggest "problems" guys have with coolant changes is not taking the appropriate steps to allow the heater core circuit to self bleed!

If you follow appropriate procedures, which includes filling the system slowly to minimize air entrapment and allow the ENTIRE system to self bleed including ensuring that coolant flows through the heater circuit, you should not have subsequent problems due to insufficient coolant level.

Duke