Need help with heat, children are freezing!

[R_W]

OK so I rebuilt the heater, new core, new hoses, new motor, new fan. Flushed radiator. Coolant is full, new 195 thermostat. hose attachments match diagrams posted. Late 70's 350.

I attempted to burp the heater core by pulling off the outlet hose and letting the truck warm up. When the thermostat started to open, a weak flow came out, then would stop. Then a little more, then would stop. I kind of expected a steady flow...

Anyway, fan works, blows right direction, but always blows cold. Air box flapper works. Inlet hose gets hot, outlet hose never seems to get hot. It is acting like it still has an air lock, but not sure.

3 core radiator, system cools really good, even on hot days in summer.

Is it too cold for my thermostat to open up enough?

And just in case DCS is cruising the website, I do not drive the children around without car seats