This probably sounds dumb, but I'll ask anyway. I recently finished a head gasket job on my truck, while it was apart I drained all the coolant, changed the thermostat and water pump and some hoses and had the radiator tested for flow and then flushed. The temperature gauge on the truck reads normal, but when I hand tested the radiator, the lower section was cold and the upper half was barely warm, also the lower radiator hose to the thermostat was cold. It's been averaging just below freezing temps where I live and I dont drive far or in traffic so is it possible that the thermostat has not opened yet? If so how are the air pockets gonna get to the top of the radiator to bleed?