Which year is the van?

Looks like air pocket to me but if the engine is not overheating! It's no
air pocket for sure.
Bleed the front again!
Monitor the engine temp at all time.

Drive the van for a wile, after let the van idle for at least 20 minutes,
blow the rear heater fan, if hot at first and cool/warm  after a wile you
have an open thermostat! If open the engine will circulate the coolant in
the front for ever and at 0C it is somewhat normal to have cold coolant if
it's alway's circulate.

http://www3.sympatico.ca/huotx/front_radiator.htm
http://pages.infinit.net/engine/cooling_system_flow.htm
Cheers, Ben