i believe it was a big air bubble in the system
I did an overnight soak on the heater core with rad cleaner...flushing the hell out of it before and after.....warmed up the truck and went thru the refill process.....went on a test drive and ZERO heat !
I pulled the cap and the water did drain down maybe a qt or 2 ......but it did that before (stat opening) .
topped it off and drove it later on---FULL heat---all vents. Its never had great heat like now .
I know i will eventually have to replace blend doors though....