What I found works good is to remove the housing and thermostat. Then fill the hole with coolant until it'* right to the top. The put the thermostat in. That way you have no chance of a air pocket under that will prevent it from opening.
The heat exchanger for the passenger compartment is about the highest point in the cooling system. So any air can end up there. Ensure you have no air in the system. Fix the leaking elbow first. Chances are the O ring is leaking. Make sure you put a little lube on the O ring before you put it in.