2000 No interior heat (temp ok)

Working through some issues with a familiy van. Heat comes first!

129k miles, van has no interior heat, no matter the temp dial setting. Always blows external/internal ambient temp. Engine temp is ok, just about 1/4. Head gaskets replaced around 70k(?). Fan blows on all speeds, front and back.

Was going to start basic mechanical checks today, starting with coolant level, air in the system, brass air bleeders, temp of heater core input/output hoses, temp control baffles, etc.

Ideas?

Thanks!

update: replaced the radiator cap, put in about 1/2-gal of 50/50 coolant-mix. Found the brass bleeders (right one over thermostat, left one over water pump), and after about 30min of bleeding, heat is coming out of the front. Back is still blowing cold, and the bleeders are only streaming randomly, so there must still be more air in the system.

update: repeated cycles of the engine temp, with bleeding and frequent refills of the overflow bottle, has heat coming out of front/rear. Mechanically, all appeared ok, with just enough coolant to keep the engine cool, yet not enough to keep coolant flowing through the heater cores. All appears ok now.