make sure the surge tank is full to the top mark. on my S.I.L. truck, I actually removed a hose and stuck a funnel in it and filled it that way. The engien had plenty of coolant, but the heater hoses were air locked.
Could pressure the line going and see if the return is spitting back out, usually when mine go tho they drip in the cab, and smells like antifreeze, not sure of a sure fire way to test, but they cost like $20-30, and easy enough to change figure if you replaced the rest of the system already that's the best spot left
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