The heater is a bit of a high point in the system and may be part of the problem.

Also, as soon as your pour cool coolant into the system, the thermostat will slam shut, so unless you preheated the coolant, that's going to prevent coolant from getting back up thru the thermostat into the top tank of the radiator. Just run it a bit till the thermostat opens and refill as needed.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.