I just changed the heater core, all was good for the last week. Noticed this Am coolant was down. what am I missing?

 

A leak? Just pressure test the system to find if there are others.

 

Was the heater core and O-rings the only things you changed?

 

Yep,that was it

 

I'd top off the reservoir and monitor the level. I'm thinking the system may have had some air in it from the original fill and it's "burbed" out. Also feel around the carpets just to make sure you don't have an o-ring leaking going to the heater core if it drops again.

 

I was thinking the same thing. I did my core replacement using an Airlift 50000 to get the air out of the system. If it was leaking inside, he should smell the coolant.