I had heat yesterday before I changed the coolant. I flushed the cooling system and added the new coolant today, it took four gallons and some water (not enough) so I tried to run it for a little while to get the coolant flowing through the system. It is definitely flowing, and I tried bleeding the air out of it but there doesn't seem to be any. I still don't have heat, but I am sure the thermostat is working. Any thoughts would be greatly appreciated!

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Update: truck has been running for about an hour in the drive way. It's not quite at normal operating temperature yet, but there is some heat coming out of the vents. Not nearly as much as there should be though, so I still think there isn't enough fluid in the cooling system, but I still can't get it to take any

 

There has to be air in the system